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551.49

N7MVAS

1988

S 5 . 0 0

VOLUME 1

MISSOULA VALLEY AQUIFER STUDY:
HYDROGEOLOGY OF THE EASTERN PORTION OF THE MISSOULA AQUIFER,

MISSOULA COUNTY, MONTANA

Prepared for

Water Development Bureau
Montana Department of Natural Resources and Conservation

Helena. Montana

William W. Woessner

Associate Professor

Department of Geology

University of Montana

Missoula, Montana 59812

Sr/ITF nnrnMrNTS COLLECTION
OCT 0 2 2005

MONTANA STAIE LIbKARY

1515 E. 6th AVE.
.yELENA. MONTANA 59620

wcrcR necE
LINE OF EQUAL ELEVATION (FT.)

Figure S.2: Water table aap for June 1986. Arrow* indicate general direction of ground water flow.

December 16, 1988

Montana State Librarv

, ii*^^^^^^^^

3 0864 1003 8325 9

6002 6 z aio

MISSOULA VALLEY AQUIFER STUDY:
HYDROGEOLOGY OF THE EASTERN PORTION OF THE MISSOULA AQUIFKR,

MISSOULA COUNTY, MONTANA

Prepared for

Water Development Bureau
Montana Department of Natural Resources and Conservation

Helena, Montana

William W. Woessner

Associate Professor

Department of Geology

University of Montana

Missoula, Montana 59812

December 16, 1988

Digitized by the Internet Archive
in 2013

http://archive.org/details/missoulavalleyaq1988woes

EXECUTIVE SUMMARY

INTRODUCnOH

The people of the Missoula Valley have used both surface and ground water
for their municipal water supply since the mid-1900's. Rattlesnake Creek
supplied surface water, and wells drilled into the valley floor have provided
ground water. In the summer of 1983 giardia contamination forced the
abandonment of the Rattlesnake Creek water supply system and valley residents
became dependent solely on ground water, which comes primarily from the
Missoula Aquifer.

The aquifer is unconfined and composed primarily of a 100 to 150 ft thick
sequence of sand, gravel and cobbles. Below this surficial aquifer are over
2,000 ft of fine grained sediments overlying bedrock. The absence of any
highly productive aquifers below the Missoula Aquifer together with the coarse
grain size of the sediments at the top of the aquifer accent the need for
careful management of this resource. This management must sustain sufficient
water quantity and good water quality to meet the requirements of the Missoula
area.

GOALS AND OBJECTIVES

This study has two goals. The first goal is to provide a scientific
foundation upon which short and long terra resource management decisions can be
made. The second goal is to build on the pre-1985 data base to provide
background for measuring existing anthropogenic effects and for assessing the
potential for future problems. Specific objectives include:

1 . Description of the physical properties of the Missoula Aquifer;

2. Initiation of a long term water level monitoring system for the

aquifer;

3. Documentation of the seasonal variations in the water table;

4. Delineation of the areas recharging the aquifer, the general

direction of ground water movement within the aquifer, and the
locations of aquifer discharge areas;

5. Quantification of the volume of water recharging the aquifer and

the volume removed by pumping and natural discharge;

6. Description of the chemical properties of the ground water and

documentation of existing and potential sources of
contamination; and

7. Development of a computerized numerical model of the aquifer to

assess the impacts of natural and induced variation in recharge
to the aquifer and in increased ground water removal by pumping.

STUDY RESULTS

A. Aquifer Stratigraphy

The Missoula Aquifer is apparently composed of three llthologlc units
which are identifiable throughout a large part of the aquifer. All contain

sand and gravel but have other features which determine their hydrologic
character. The top unit (Unit One) is 10 to 30 ft thick, bouldery and
generally lies above the saturated zone of the aquifer. The middle zone (Unit
Two) is 40 ft thick and appears to have reduced water transmitting capacity
due to its fine grained nature. The basal unit, Unit Three, iS 50 to 100 ft
thick, very coarse and currently yields large quantities of water to the many
wells developed in it. Depending on how fine the grain size of Unit Two is,
Unit Two's presence above the productive basal zone may protect underlying
portions of the aquifer surface sources of contamination.

B. Hydraulic Properties

Overall, the Missoula Aquifer has a porosity of 0.20, a specific capacity
of 0.12, a hydraulic conductivity of 18,200 gpd/ft^ and a transmissivity of
1,152,000 gpd/ft. These values vary spatially depending on the presence and
thickness of Unit Two and the thickness of the saturated portion of Unit One.
As an example, hydraulic conductivity values are as high as 25,500 gpd/ft^ for
Units One and Three and about 8,000 gpd/ft^ for Unit Two. Hydraulic
conductivity and transmissivity values are apparently extremely high for Unit
Three. Large production wells pump water from this unit with little drawdown.

C. Variations in Water Table Position

Five significant trends in the water table position are observed. The
first is an annual water level rise which occurs between about March and June,
and then a general decline until the following February or March. Second, the
elevation of the spring peak water level at all wells decreased in 1987 from
1986. Minimum water table elevations were also lower in 1987 than 1986.
Third the seasonal water table fluctuations decrease with increasing distance
from the Clark Fork River or other influent streams. Fourth, wells located in
the Missoula Aquifer, near the mouths of Grant Creek and possibly Rattlesnake
Creek, have the largest annual fluctuations in water table elevation.
Finally, the ground water system is also affected by flow in the Bitterroot
River, as indicated by wells located near the Bitterroot River. The maximum
water table elevation at these wells coincides with the spring runoff peak
stage of the Bitterroot River.

Analysis of the long term water level trends in the valley indicate the
effect of climatic influences and pumping stress on the aquifer. Hydrographs
illustrate the longer term trend towards a general lowering of the valley
water table since 1983. This trend is observed as a decrease in the
elevations of yearly water table maxima and minima between 1983 and 1987.
This record is too short to adequately assess whether a long term trend of net
annual decline is actually occurring. This apparent lowering of the water
table can be attributed to increases in withdrawal by Mountain Water Company
starting in 1983 and to decreases in the Clark Fork River annual and peak
discharges which also began in 1983 and continued through 1987.

D. Ground Water Movement and Recharge-Discharge Relationships

The sai^e general pattern of ground water flow is observed through most of
the year. North of the Clark Fork River, ground water moves away from the

channel and away from the northern aquifer boundary, where ground water from
the Tertiary sediments and the alluvium in the Grant Creek and Rattlesnake
Creek Valleys recharges the aquifer. The net result of recharge from these
different sources is ground water migration parallel to the river channel and
final discharge to the Clark Fork River north of the river's confluence with
the Bitterroot River. South of the river ground water flows southwest towards
the Bitterroot River and towards the confluence of the Clark Fork and
Bitterroot Rivers.

Water recharges the Missoula Aquifer by leakage from the Clark Fork
;River, direct precipitation on the aquifer, inflow from the adjacent Tertiary
Sediment and Bedrock Hydrostratigraphic Units, storm water runoff, septic
system percolation and leakage from irrigation ditches. Leakage from the
seven mile reach of the Clark Fork River in the valley constitutes over 90% of
the recharge to the Missoula Aquifer.

Water discharges from the aquifer by evapotranspiration, as base flow to
streams and by pumping. Evapotranspiration rates are quantified and assumed
small. Ground water is discharged by pumping of wells owned by individuals,
industrial users, and two private water companies. Mountain Water Company and
Clark Fork Water Company (CFWC). On the average, these wells pump greater
than nine billion gallons per year from the aquifer. The volume of natural
discharge from the Missoula Aquifer was approximated (assuming no net change
in storage) by subtracting estimated annual recharge to the aquifer from
annual withdrawal from wells. Estimated natural discharge is 93% of the
annual recharge.

Aquifer recharge exceeds the estimated ground water withdrawal by 15
times. The leakage of water through the bed of the Clark Fork River accounts
for over 90% of annual recharge. Therefore, observed water table declines
most likely reflect recent climatic changes which have caused reductions in
recharge rates from the river and Tertiary sediments.

E. Water Quality

The ground water in the Missoula Aquifer is calcium bicarbonate water.
Near the Clark Fork River, mineral content of ground water fluctuates
seasonally in response to changes in water quality of the river. Throughout
the rest of the aquifer, mineral content remains essentially constant at any
one point but increases in the direction of ground water flow. Water is ,:
generally good quality and meets drinking water standards.

Water quality steadily decreases in the down gradient direction from the
Hellgate Canyon area east and southeast toward the Bitterroot River. For
example, total dissolved solids increase from a low of about 240 mg/1 to a
maximum of 358 mg/1 over distances of four to five miles. This down gradient
increase in total dissolved solids is probably caused by natural dissolution
in the aquifer of carbonate minerals, such as calcite and dolomite. The
increase in calcium, magnesium and bicarbonate concentrations correlates with
the increasing levels of total dissolved. Concentrations of other major ions,
such as sulfate, sodium, and chloride, do not increase consistently with
increasing total dissolved solids.

i i i

Concentrations of chloride and nitrate also Increase west and south west
from the Clark Fork River along paths of ground water flow. These two Ions
are important Indicators of pollution. Though concentrations do not exceed
drinking water standards, above background values In Isolated portions and In
much of the western portion of the aquifer indicate ground water degradation
occurs in these areas. The probable sources of contamination are wastes from
seepage rings and septic tanks and from road salt In commercial areas.
Testing of 98 wells for collform bacteria resulted In 18 positive tests, an
indication septic system waste is entering the ground water system. In
addition to widespread changes in water quality, a number of Isolated events,
such as Improper herbicide disposal, have affected portions of the aquifer and
illustrate the vulnerability of the aquifer to pollution.

F. Ground Water Flow Modeling

Significant progress was made in defining the distribution of aquifer
properties and boundaries and in understanding the Interaction between the
Clark Fork River and the ground water system. Efforts to construct and
calibrate a transient model of ground water flow through the Missoula Aquifer
were unsuccessful. Future modeling efforts will be used to describe the
responses of the aquifer to variations In Clark Fork River recharge and to
pumping by Mountain Water Company; additional data are required to develop the
next model.

CONCUJSIONS AND RECOMMENDATIONS

The results of this two year study lead to the following conclusions:

1. The Missoula aquifer is stratigraphlcally complex. It is composed
of three units, the second of which is not always present. The upper
most unit. Unit One, is 10 to 40 ft thick and is composed of Interbedded
boulders, cobbles and gravel. The middle zone. Unit Two, is composed of
up to 40 ft of tan to yellow silt with sand and gravel. Unit Three, the
basal unit, is composed of 50 to 100 ft of Interbedded gravel, sand and
silt.

2. The hydrologic properties of the aquifer reflect site specific
stratigraphy and depositional environments. Values of hydraulic
conductivity and transmissivity appear to decrease southwest of the
Hellgate Canyon and Grant Creek area. Approximate hydraulic properties
of the entire aquifer, assuming the aquifer is acting as one homogeneous
unit, are a porosity of 0.20, specific yield of 0.12, hydraulic
conductivity of 18,200 gpd/ft^ and a transmissivity of 1,152,000 gpd/ft.

less than normal Clark Fork River spring discharge and aquifer recharge.
Also since 1983, the main water producer in the valley, Mountain Water
Company, more than doubled its ground water pumping rate. As a result,
a general lowering of the water table occurred at Mountain Water Company

wells .

4. The Clark Fork River is a losing stream and seasonally recharges the
aquifer over a four to six mile reach. Mass balance calculations show
that the river accounts for 90 percent of aquifer recharge. Total
aquifer recharge is 15 times greater than withdrawal from Mountain Water
Company wells, Clark Fork Water Company wells and approximately 4,700
individual wells. Based on these data, the apparent valley wide decline
in the water table since 1983 is a result of a reduction in recharge
caused principally by lower than normal flow in the Clark Fork River.

5. North of the Clark Fork River, the direction of ground water flow is
strongly influenced by recharge from the boundary foothills on the north
and from the Clark Fork River. Flow in this part of the aquifer
parallels the river and moves west until it passes the Reserve Street
area, where it turns south to discharge to the Clark Fork River below
its confluence with the Bitterroot River. In the area of Missoula south
of the Clark Fork River, ground water flows south and southwest to
discharge to the Bitterroot River and, seasonally, to the lower reaches
of the Clark Fork River.

6. The water quality of the aquifer is good and does not require
treatment prior to use in most areas. The water is dominated by calcium
and bicarbonate ions, is low in total dissolved solids (less than 500
mg/1) and is similar in chemistry to the Clark Fork River. The presence
of chloride and nitrate concentrations which are elevated above natural
background levels suggests degradation from anthropogenic sources is
occurring. Injection of storm water was found to increase chloride
concentrations at the water table beneath storm drains. The disposal of
sewage by seepage rings or drain fields appears to be degrading water
quality in portions of the valley as both nitrate and chloride
concentrations are elevated in areas served by septic systems.
Fortunately, concentrations do not exceed drinking water standards.
However, coliform bacteria and fecal coliform bacteria have been found
in individual water supplies in numbers which exceed drinking water
criteria. The aquifer is extremely vulnerable to contamination because
it is unconfined and consists of generally coarse material.

7. An attempt to numerically simulate the aquifer's complex
stratigraphy and recharge-discharge relationships was unsuccessful. The '^
computer model could not be calibrated using field data and then
independently verified with a second set of field data. Problems were
viewed as being partially attributable to inadequate definition of *
recharge rates from the Clark Fork River and of the distribution of
aquifer properties.

The conclusions listed above imply that the Missoula Aquifer is an
aerially extensive, thin, unconfined system which is geologically complex and

vulnerable to contamination. It is Missoula's sole source of water and an
irreplaceable resource. Protection of the Missoula Aquifer requires
continuing long term observation and management. The following
recommendations are based on the conclusions outlined above:

1. Maintain a long term water level observation network.

It is the goal of these recommendation to provide the citizens of Missoula
with the facts needed to make educated management decisions regarding the
future of their source of potable water. Halting data collection and further
work on model development would be short sighted and could result in poor
planning and crises in managing this critical resource.

ACKNOWLEDGMENTS

Though this work is authored by one person it could not have been
completed without the contributions of many individuals and groups. David
Nimick provided final editorial work and his effort is greatly appreciated.
Researchers Bill Clark, Bill Morgan, Mike Pottinger, Karen Wogsland, Bill
Peery and Ross Miller contributed significantly to the project. Jim Bigley
assisted with instrumentation and Bill Thompson, Linda Angeloni and Cas Smith
helped with data processing. Preliminary research results were also discussed
with the staff of the Missoula City County Health Department and local
hydrogeologists and hydrologists.

This project received invaluable support from the Missoula City County
Health Department, Mountain Water Company, the Missoula County Surveyors
Office and hundreds of residents of Missoula who allowed access to their
property and wells.

The citizens of Montana funded about one half of this effort through the
Water Development Grant Program of the Montana Department of Natural Resources
and Conservation. The Missoula County Commissioners, through the Missoula
City County Health Department, also assisted significantly with financial
support. Additional research funds from the US Environmental Protection
Agency, the Montana Water Resources Research Center and the University of
Montana's Excellence Fund supported portions of this effort.

1 sincerely appreciate all support.

TABLE OF CONTENTS

Page

EXECUTIVE SUMMARY i

ACKNOWLEDGMENTS vii

LIST OF FIGURES , x

LIST OF TABLES xlv

LIST OF PLATES xv

LIST OF APPENDICES xvi

CHAPTER I : INTRODUCTION 1

CHAPTER 2: STUDY AREA SETTING 3

Physiography and Geomorphology 3

Climate , 3

Surface Water Hydrology 3

Geology 7

Ground Water Hydrology 11

Water Quality 14

CHAPTER 3: GEOMETRY, STRATIGRAPHY AND HYDROLOGIC PROPERTIES OF

THE MISSOULA AQUIFER 16

Introduction 16

Aquifer Geometry 16

Aquifer Stratigraphy 17

Hydrologic Properties 27

Results of Aquifer Parameter Determination 28

Summary of Hydrologic Properties 30

CHAPTER 4 : WATER TABLE FLUCTUATIONS 34

Monitoring Well Network 34

Results of Water Level Monitoring 37

Historical water Level Trends 51

CHAPTER 5: GROUND WATER FLOW: SOURCES OF RECHARGE AND DISCHARGE 62

Ground Water Flow System 62

Aquifer Recharge and Discharge 68

CHAPTER 6 : AQUIFER WATER QUALITY 73

Methods 73

Results: General Description of Ground Water Quality 73

Chemical Trends: Results of Spring 1987 Sampling 79

Examples of Ground Water Contamination 85

CHAPTER 7 : GROUND WATER FT.OW MODELING lOA

Introduction 104

Model Design 104

Transient Flow Simulation • Ill

Model Results 114

CHAPTER 8 : CONCLUSIONS AND RECOMMENDATIONS 121

REFERENCES 125

LIST OF FIGURES

Figure Page

2.1 Location of the study area and the eastern two-thirds of

the Missoula-Ninemile Valley (Huson to Missoula) A

2.2 Yearly precipitation totals, 1956 to 1968 5

2.3 Location of the USGS surface water gaging stations 6

2 .4 Average and monthly flow for the Clark Fork River at Bandman

Bridge above Missoula 8

2.5 Average and monthly flow for the Clark Fork River at the USGS

gage below Missoula 9

2.6 Geologic map of the study area 10

2.7 Schematic north-south cross-section of the Missoula Valley 12

3.1 Well logs and interpreted contact between Quaternary and

Tertiary sediments 18

3.2 Interpreted depth to the base of the Missoula Aquifer 19

3.3 Location of geologic cross sections , 20

3.4 Cross section A-6 21

3.5 Cross section C-D 22

3.6 Cross section E-F 23

3.7 Cross section G-6 24

3.8 Distribution of hydraulic conductivity calculated from

specific capacity data 31

3.9 Distribution of transmissivity calculated from specific

capacity data 32

4.1 Location of project monitoring network wells and Mountain

Water Company wells 35

4.2 Location of monitoring wells constructed for this project 36

4.3 Location of wells equipped with water level recorders 38

4.4 Hydrographs of data from wells MV34 and MV35 39

4.5 Hydrograph of data from well MV6 40

Figure Page

4.6 Hydrographs of data from wells MV31 and MV26 41

4.7 Hydrograph of data from well MVIO 42

4.8 Hydrographs of data from wells MV39 and MV20 43

4.9 Hydrographs of data from wells MV2 and MV36 44

4.10 Hydrograph of data from well P31 45

4.11 Hydrographs of transducer data from wells MV34 and MV35 46

4.12 Hydrographs of transducer data from wells MV37 and MV31 47

4.13 Hydrographs of transducer data from wells MV40 and MV39 48

4.14 Hydrographs of transducer data from wells MV38 and MV36 49

4.15 Hydrographs of data from well MV34 and the Clark Fork River

at the University Walking Bridge 52

4.16 Hydrographs of data from well MV37 and the Clark Fork River

at the Reserve Street Bridge 53

4.17 Hydrographs of data from the Clark Fork River and wells

located progressively further from the river 54

4.18 Hydrographs of data from well MV39 and the Bitterroot River

at Buckhouse Bridge 55

4.19 Total monthly ground water withdrawals by Mountain Water

Company 56

4.20 Hydrograph of data from well MV31 (MWC7) 58

4.21 Hydrographs of data from wells MWC34 and MWC30 59

4.22 Hydrographs of data from wells MWC19 and MWC20 60

4.23 Hydrographs of data from well MV31 (MWC7) and the Clark Fork

River showing the departure from the average monthly discharge... 61

.5.1. Water table map for March 1986 63

5.2 Water table map for June 1986 64

5.3 Water table map for October 1986 65

5.4 Clark Fork River channel profile and the corresponding

elevation of the water table 66

Figure Page

5.5 Changes in the location of the 3,135 ft equipotential line

during the water table decline from June 1986 to March 1987 67

5.6 Percentage of discharge, measured at the Walking Bridge,
recharging the aquifer over a three mile reach of the river 69

5.7 Calculated average monthly loss from the Clark Fork River 70

6.1 Project water quality monitoring network 74

6.2 Location of water quality sampling sites for the spring 1987
monitoring 75

6.*^ Distribution of major cations and anions, November and

December 1986 77

6 .A Hydrographs showing seasonal variation in TDS for the Clark

Fork River and adjacent wells and seasonal change in TDS along

a ground water flow path from CFNWB to MV20 78

6.5 Distribution of total dissolved solids, spring 1987 80

6.6 Distribution of chloride, spring 1987 82

6.7 Distribution of nitrate, spring 1987 83

6.8 Location of positive coliform bacteria tests, spring 1987 84

6.9 Location map for the herbicide study at the Missoula County

Weed Control Facility 86

6.10 Schematic diagram of sump at Missoula County Weed Control
Facility and results of water quality and soil chemical

analyses 87

6.11 General area in which measurable concentrations of picloram

and bromacil were detected in the ground water. 88

6.12 Modeled distribution of herbicides in 1974 90

6.13 Modeled distribution of herbicides in 1989 91

6.14 Schematic diagram of a Missoula storm drain and hydrogeologic
instrumentation 92

Figure Page

6.15 A comparison of water quality data from runoff, vadose water
and ground water at a residential and a commercial storm drain

site 93

6.16 Chloride concentration and water table fluctuations versus

time for a commercial site and residential site 94

6.17 Iron concentration and water table fluctuations versus time

at a commercial site and a residential 95

6.18 Nitrate concentration and water table fluctuations versus

time at a commercial site and a residential site 96

6.19 Location and instrumentation of septic system study sites 98

6.20 Schematic diagram of hydrologic instrumentation at each site 99

6.21 Concentration of chloride in the aquifer 100

6.22 Concentration of nitrate in the aquifer 101

6.23 Location of ground water samples contaminated with fecal

coliform bacteria 102

7.1 Grid for finite difference model 105

7.2 Model grid and location of Mountain Water Company wells... 106

7.3 Model boundary conditions and locations of wells used to

evaluate calibration 107

7.4 Distribution of hydraulic conductivity used in the model 109

7.5 Elevations of the base of the aquifer used in the model 110

7.6 Hydrographs showing average monthly stage used in the model for

the Clark Fork River and the Bitterroot River 112

7.7 Modeled and measured water table elevations, October 1985 115

7^8 Modeled and measured water table elevations March 1986 116

7.9 Modeled and measured water table elevations, June 1986 117

7.10 Hydrograplis showing modeled and measured heads at wells MV3

and MV31 , 118

7.11 Hydrographs showing modeled and measured heads at well MV22 119

xni

LIST OF TABLES

Table Page

2.1 Hydrostratigraphlc units of the Missoula Valley..... 13

3.1 Stratigraphic zones of the Missoula Aquifer 25

3.2 Grain size data from sieve analyses for Unit One of the

Missoula Aquifer 26

3.3 Estimates of aquifer properties 28

4.1 Net water table rises from winter to spring peak 50

5.1 Volume estimates of recharge to the Missoula Aquifer 72

5.2 Water use from the Missoula Aquifer 72

6.1 Water analyses of representative Missoula Valley ground

water samples collected February 5, 1986 76

6.2 Average water quality for all water analyses 76

6.3 Results of EPA priority pollutant scans 85

6 .A Septic system treatment as a mean percent removal of effluent

constituents 97

7.1 Schedule of injection wells used to simulate Rattlesnake

Creek and Grant Creek 108

7.2 Pumping schedules for Mountain Water Company municipal wells..... 113

7.3 Leakage values used in the model 114

XIV

LIST OF PLATES

Plate

1 Air photo map showing well locations In pocket

LIST OF APPENDICES

VOLUME 2

Appendix Page

3A Well Inventory 128

3B Computer Program PTRAN 161

3G. Transmissivity , Hydraulic Conductivity and Specific Capacity 166

4A Well Logs of Project Installed Monitoring Wells 169

4B Water Level Measurements 179

4C Mountain Water Company Monthly Total Production 236

5A Clark Fork River Leakage Calculations , 243

6A Water Sampling Results 249

6B Spring 1987 Sampling Results 260

6C Results of Organic Analyses 273

XVI

CHAPTER 1
mTRODUCnON

The people of the Missoula Valley have used both surface and ground water
for their municipal water supply since the raid-1900's. Rattlesnake Creek
supplied surface water and wells drilled into the valley floor have provided
ground water. In the summer of 1983 giardia contamination forced the
abandonment of the Rattlesnake Creek water supply system and valley residents
became dependent solely on ground water, which comes primarily from the
Missoula Aquifer.

The Missoula Aquifer consists of 50 to 150 feet (ft) of coarse sand and
gravel which directly underlie the valley bottom. Currently the aquifer
supplies over 9.7 billion gallons of water annually. The valley is surrounded
by hills and mountains where bedrock and fine grained Tertiary aquifers
produce only limited quantities of ground water. Therefore the Missoula
Aquifer is considered the valley's primary source of water. In June 1988 the
aquifer met EPA sole source aquifer criteria. Currently, water from the
Missoula Aquifer requires no treatment and the supply of water appears
adequate. However the ability of the ground water system to sustain continued
development and resist wide spread contamination is unclear.

This study has two goals. The first is to provide a scientific
foundation upon which short and long term resource management decisions can be
made. The second is to build on the pre-1985 data base such that existing
impacts can be measured and the potential for future problems can be assessed.
Specific objectives include:

1 . Description of the physical properties of the Missoula Aquifer;

2. Initiation of a long terra water level monitoring system for the

aquifer;

3. Documentation of the seasonal variations in the water table;

4. Delineation of the areas recharging the aquifer, the general

direction of ground water movement within the aquifer, and the
locations of aquifer discharge areas;

5. Quantification of the volume of water recharging the aquifer and

the volume being removed by pumping and natural discharge;

6. Description of the chemical properties of the ground water and

documentation of existing and potential sources of
contamination; and

7. Development of a computerized numerical model of the aquifer

capable of assessing the impacts of natural and induced variation
in aquifer recharge and increased ground water pumping.

During its two year time span, this large project was divided into a
number of smaller studies. All have been under the direct supervision and
direction of the author. They include graduate and undergraduate thesis
research at the University of Montana by Meyer (1985), Clark (1986), Morgan
(19860, Sendler (1986), Bayuk (1986), Peery (1989), Pottinger (1988), Wogsland
(1988), and Ver Hey (1987). Ground water modeling was performed by Pottinger

(1988) and research associate Brick (1987). These recent works rely upon the
earlier work of Geldon (1979) and McMurtrey and others (1965). Many of these
studies are summarized in this report. The reader is urged to consult these
referenced reports for additional detailed information. Other portions of the
study have not been published as separate reports. Pertinent data from these
project activities are included in the appendices.

This report is organized into two volumes. Volume 1 contains the main
text, figures and tables. Volume 2 is the appendix to the report.

The chapters in Volume 1 focus on the seven specific objectives listed in
the beginning of this chapter. Chapter 2 briefly describes the study site.
Chapter 3 addresses the geometry, stratigraphy and hydro logic properties of
the aquifer. CUapter A describes the ground water monitoring program used in
this project and reports on the observed seasonal fluctuations of the water
table. Chapter 5 quantifies aquifer recharge, aquifer discharge and
directions of ground water flow. Chapter 6 describes water quality in the
aquifer and summarizes three case studies of aquifer contamination. Chapter 7
describes the attempt at modeling the physical ground water system in an
effort to evaluate future effects on the aquifer. Chapter 8 presents
conclusions and recommendations.

Volume 2 contains supporting material for chapters three, four, five and six.

CHAPTER 2
STUDY AREA SETTING

Numerous authors have prepared general site descriptions of the Missoula
Valley. The one presented here is taken primarily from the work by Clark
(1986). ,- ,

PHYSIOGRAPHY AND GEOMORPHOLOGY

The Missoula Valley is approximately 35 ml^ in size. It is bounded by,'
the Rattlesnake Hills on the north, the Sapphire Mountains on the east, the -
Bitterroot Mountains on the south and, on the west, by a low plateau in the
vicinity of the Johnson-Bell airport (Plate 1). The Missoula Valley is the
eastern part of a long intermontane depression trending WNW from the city of
Missoula (Figure 2.1).

The topography of the valley floor is relatively flat, but slopes
gradually downward from the bounding highlands towards the northwest, where
the Clark Fork River leaves the valley. There are two major river terraces on
the valley floor. Twenty foot scarps separate the terraces from each other
and from the floodplains of the Clark Fork and Bitterroot Rivers (Geldon,
1979). Another valley floor feature is McCally Butte, a 200 ft high bedrock
outlier located in the floodplain of the Bitterroot River.

The Missoula Valley is drained by the Clark Fork and Bitterroot Rivers.
The Clark Fork River enters the valley from the east through the 1,500 ft deep
Hellgate Canyon (Plate 1). The river flows westward for about seven miles,
meeting the Bitterroot River at Kelly Island. The Clark Fork River has a
gradient of 10. A ft/mi. The Bitterroot River enters the valley in the south
central portion of the study area. It flows northwest for 4.5 mi at a
gradient of 5.2 ft/mi before joining the Clark Fork River. Several smaller
streams enter Che valley from the surrounding highlands, including Rattlesnake
Creek, Grant Creek and Pattee Creek (Plate 1).

CLIMATE

The climate of the Missoula Valley is semiarid. Winter is dominated by
Pacific maritime air, which occasionally is displaced by cold continental air
draining through the Clark Fork Valley. Annual precipitation averages 13.29
inches (NCAA, 1987) (Figure 2.2). Peak precipitation occurs in May and June.
The least precipitation occurs in February and March. High intensity
convective storms in July and August may also contribute significant
precipitation.

SURFACE WATER HYDROLOGY

The Clark Fork River is gaged above and below Missoula by the U.S.
Geological Survey (USGS) (Figure 2.3). The gaging station above Missoula is
located near Bandman Bridge, 2.8 mi east of Missoula. The station below

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Missoula is located I .0 mi dovmstream of the confluence with the Bitterroot
River, 4.5 mi west of Missoula. Both have been gaged continuously since 1929.

The drainage area of the Clark Fork River above the upper gaging station
is 5,992 mi^ . Based on 57 years of data (1930 to 1986), mean annual discharge
is 3,051 cubic feet per second (cfs) (USGS, 1987) (Figure 2.4). The maximum
recorded daily flow was 32,300 cfs on June 21, 1975. The minimum flow of 340
cfs occurred on September 27, 1937. Average monthly discharges indicate the
general response of the river to spring runoff. The lowest mean monthly flow
(1,596 cfs) occurs in December while spring rains and snowmelt cause the
maximum mean monthly flow (8,740 cfs) in June (Figure 2.4). Peak discharge in
1985 was 8,680 cfs on May 25 and minimum discharge was 826 cfs on August 1.
In 1986, the river peaked at 12,900 cfs on June 1 and had its minimum
discharge of 979 cfs on August 20 and 21.

The gaging station below Missoula records the combined flow of the Clark
Fork and Bitterroot Rivers. Mean annual discharge is 5,547 cfs (USGS, 1984)
(Figure 2.5). A maximum discharge of 52,800 cfs occurred on May 23, 1948; the
minimum was 388 cfs on January 18, 1933. Mean monthly discharge varies from
2,045 cfs in December to 20,160 cfs in June (Figure 2.5). A 1985 peak
discharge of 19,900 cfs occurred on May 25; the minimum discharge was 1,180
cfs on August 1. In 1986 a peak of 32,100 cfs was observed on May 31 and a
minimum flow of 1,620 cfs on August 21. The Bitterroot River contributes
about 3,000 mi^ to the 9,003 mi^ drainage area above this gage.

The Bitterroot River was gaged independently from 1898 to 1905 at a
bridge four miles southwest of Missoula. For this period of record, mean
annual discharge was 3,260 cfs (USGS, 1975). Geldon (1979) estimates the mean
annual discharge for the Bitterroot River as 2,339 cfs, or about 40% of the
total flow of the Clark Fork River below the confluence of the rivers.

Rattlesnake Creek was gaged from 1959 to 1967 at the Vine Street Bridge
in Missoula. The drainage area is 79.7 rai^ . Mean annual discharge for the
period was 110 cfs (USGS, 1975). Geldon (1979) estimated the mean annual
discharge for the period from 1959 to 1977 as 135 cfs by extrapolating the
Rattlesnake Creek gaging data with Clark Fork River gaging records for the
entire period. ,

Pattee Creek and Grant Creek have not been gaged by the USGS. Geldon
(1979) estimated discharge for Pattee Creek by comparing drainage area and
precipitation data for these ungaged drainages with Rattlesnake Creek data.
Mean annual discharge is 13 cfs for Pattee Creek, du Breuil (1983) gaged
Grant Creek and estimated the mean annual discharge at 30 cfs,

GEOLOGY

The sediments of the Missoula Valley are continental clastic deposits
(Figure 2.6). The floor of the valley is covered by alluvial and lacustrine
sediments of Quaternary age, 1.6 million years to the present. These deposits
are described in more detail in Chapter 3,

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ANNUAL FLOW

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Fork. River at the USGS gage below Missoula.

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The low foothills surrounding the valley floor are principally composed
of fine grained sediments deposited when the basin was internally drained
during the Tertiary period, 43 to 5.3 million years. Although up to 2,500 ft
of Tertiary sediments are preserved in the valley (McMurtrey and others,
1965), surface exposures are infrequent and poor around the valley margins.
The sediments range in size from clay to coarse gravel and unconf ormably
overlie Precambrian Belt Supergroup metasediments. Tertiary sediments are
subdivided into three stratigraphic units:

1. Fanglomerates (pre-Renova Formation equivalent) which are believed

to be limited to the basin's margins, ? to 43 million years;

2. Fine grained ash-rich sediments (Renova Formation equivalent) which

underlie the valley floor, 43 to 20 million years; and

3. Coarse clastic sediments (Sixmile Creek Formation equivalent) which

are found locally in the foothills and may be overlying the
Renova Formation beneath the valley floor, 20 to 5.3 million
years .

A north-south diagrammatic section of the valley showing Tertiary and
Quaternary deposits is presented in Figure 2.7. The Cenozoic geology of the
Missoula area has been described in detail by McMurtrey and others (1965),
Kuenzi and Fields (1971), Fields (1981), Thompson and others (1982),
Wehrenberg (1983) and Fields and others (1985).

Mountain ranges surrounding the valley, including Mount Jurabo and Mount
Sentinel, are composed of Precambrian metasediments of the Belt Supergroup,
0.8 to 1.6 billion years.

The interraontane depression, of which the Missoula Valley is part, is
believed to have formed as a result of horizontal extension after Laramide
thrusting, which occurred between late Cretaceous and middle Eocene time, 97.5
to 52 million years ago (Fields and others, 1985). The horizontal extension
resulted in normal faulting parallel to the faces of Mount Jumbo and Mount
Sentinel and in the formation of the Clark Fork Fault, which is exposed on the
north side of the valley (Figure 2.6).

GROUND WATER HYDROLOGY

Missoula Valley residents use ground water from three sources: fractured
Precambrian Belt Supergroup rocks. Tertiary Renova equivalent sediments, and
the coarse alluvium which is exposed at the surface of the valley floor (Table
2.1). Use of the fractured bedrock and the Renova equivalent aquifers is
generally restricted to the valley margins. The shallow sand and gravel
deposits below the valley floor are the principal source of ground water.

The Precambrian bedrock surrounding the valley is generally a poor
aquifer. The rock is essentially impermeable and water flows only through
fractures. Wells generally yield about one gallon per minute (gpm) . However,
in the Hayes Creek area, a well yielded 17 gpm during a four hour aquifer test
(Bayuk, 1986). Wells finished in bedrock range in depth from 38 to over 1,000
ft. Geldon (1979) reported an average specific capacity of 0.11 gallons per
minute per foot of drawdown (gpm/ft). Analysis of driller's reports in the

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Hayes Creek area indicates specific capacities vary from 0.5 to 35 gpm/ft
Bayuk (1986) noted that yield decreases with increasing well depth.

Table 2.1
Hydrostratigraphic Units of the Missoula Valley

Hydros t rat igraphic

Unit

Missoula Aquifer

Tertiary Sediments

Precarabrian

Age

Miocene (?)
to Recent

Late Eocene
to Early
Miocene (?)

Precambrian

Thickness
(ft)

150

2,500 to
3,500

greater

than

10,000

Description

Sand, gravel and boulders
with some silt and clay.
Clasts well rounded.
Yields up to 7,000 gpm.

Clay with interbedded and
embedded sand and gravel.
Clay is blue, gray, brown,
tan and red. Local coal
and volcanic ash. Yields
up to 40 gpm but averages
25 gpm.

Quartzite, red and green
argillite and carbonates.
Water is from fractures.
Yields up to 17 gpm, but
averages 1 gpm.

Renova equivalent sediments which occur on the valley flanks and beneath
the Missoula Aquifer locally comprise a second low yielding aquifer.
Discontinuous lenses of sand and gravel, usually less than 10 ft thick,
provide water under artesian conditions to wells, generally at less than 20
gpm (Geldon, 1979). Barclay (1986) studied the hydrogeology of the Ninemile
Valley and reported the average yield of 32 wells finished in Renova sediments
to be 11.3 gpm with a range of 0.5 to 45 gpm. Finstick (1986) found similar
values for Renova equivalent sediments in the Bitterroot Valley near Victor.
However, well MWC3 on South Avenue (Plate 1) penetrates 2 three foot zones of
sand and gravel in the Renova equivalent sediments which yielded 75 gpm when
the well was developed. Geldon (1979) reported hydraulic conductivities (K)
averaging 165 gallons per day per foot squared (gpd/ft^), specific capacities
of 0.51 gpm/ft and a storage coefficient of 10"^. Barclay (1986) reported an
average hydraulic conductivity of 300 gpd/ft^ with a range of 0.08 to 1,900
gpd/ft^ for Renova equivalent sediments in the Ninemile Valley.

Both the Precambrian and Tertiary units yield small quantities of water
to wells. They are utilized for domestic supplies where other more productive
sources are usually not available. In some small tributary valleys, saturated
alluvium yields small quantities of water (Barclay, 1986). The saturated
coarse sand and gravel material which directly underlies the valley floor is

of principle interest to the majority of water users in the Missoula basin.
These coarse sediments include Pleistocene and Recent alluvial sediments and
may also include an upper portion of the Sixmile Creek Formation. Although
these sands and gravels may range in age from Miocene to Recent and be part of
different geologic formations, they have similar hydrologic properties.
Therefore, these sands and gravels are grouped into one hydrostratigraphic
unit called the Missoula Aquifer. This aquifer yields over 9.7 billion
gallons of water annually to wells which supply the city and over 4,700
individual dwellings. Geldon (1979) reported that the Missoula Aquifer has an
average hydraulic conductivity of 5,100 gpd/ft^, specific capacities of over
3,000 gpm/ft and transmissivities (T) of over 1,000,000 gpd/ft. Aquifer
storage coefficients range from 0.11 to 0.35. It is the Missoula Aquifer that
is the focus of this report.

Geldon (1979) prepared maps of maximum and minimum water table positions
in the Missoula Aquifer. He noted seasonal fluctuations in the water table
with highs in the first half of July and lows around March 1. Both McMurtrey
and others (1965) and Geldon (1979) recognized the importance of the Clark
Fork River in recharging the surficial sand and gravel system. They
interpreted flow directions to be away from the Clark Fork River. Flow is to
the west on the north side of the river and to the southwest on the south
side. Geldon (1979) estimated the average discharge of water through the main
part of the Missoula Aquifer to be 5.1 x 10^ ft^/d at a velocity of 6.2 ft/d.
He attempted a water balance for the aquifer and concluded that discharge from
the aquifer was exceeding recharge during 1977 and 1978 by about 21,000
acrefeet (acft).

WATER QUALITY

Water quality in the Missoula Aquifer is good and well below all drinking
water standards. Juday and Keller (1978), the Montana State Water Quality
Bureau (WQB), Hydrometrics (1984) and Mountain Water Company have analyzed
water quality in the valley.

Ground water in the Missoula Aquifer is calcium-bicarbonate type. It is
moderately hard, as expressed by the sum of calcium and magnesium ion
concentrations. Total dissolved solids are usually less than 350 milligrams
per liter (mg/1). Chloride concentrations are less than 10 mg/1 and sulfate
is less than 30 rag/1. The pH ranges from 6.8 to 8.5. Geldon (1979) reported
an average calcium to silica ratio of 2.3, which implies a relatively rapid
circulation of ground water in the aquifer. The ratio decreases southwestward
away from the Clark Fork River as the ground water flows farther from the
recharge area. Seasonally, wells near the Clark Fork River and downtown
Missoula show 60% fluctuations in total dissolved solids in response to
similar changes in the Clark Fork River (Hydrometrics, 1984). The WQB
collects an annual ground water sample in the Missoula Valley.

The Mountain Water Company monitors water quality in the valley's
municipal wells. Fifty-four bacteriological samples are collected each month
(about 2 samples per well per month). Every 4 years MWC tests its wells for
inorganic chemical constituents. They also sample the Clark Fork River
periodically.

Water found in the Tertiary sediments is a calcium-bicarbonate type.
Total dissolved solids are generally less than 500 mg/1 (Geldon, 1979; Juday
and Keller, 1978). Iron concentrations typically exceed the 0.30 mg/1
drinking water standard. Based on measured calcium to silica ratios, Geldon
(1979) concluded that ground water circulated through the Tertiary sediments
longer than shallower water did in the Missoula Aquifer.

Bayuk (1986) reported water quality data for the Precambrian bedrock
aquifer underlying the Hayes Creek area. The water is calcium-bicarbonate
type with total dissolved solids ranging from 290 to 350 mg/1. Water quality
is characteristic of a bedrock aquifer near its highland recharge area. It is
anticipated that deep water below the Missoula Valley in mineralogically
similar bedrock would be considerably higher in total dissolved solids.

CHAPTER 3

GEOMETRY, STRATIGRAPHY AHD HTDROLOGIC PROPERTIES
OF THE MISSOULA AQUIFER

mTRODUCnON

One of the specific objectives of this research effort was to describe
the physical and hydrologic properties of the Missoula Aquifer. These
properties include transmissivity, storage, porosity and hydraulic
conductivity as well as the variation in aquifer thickness and internal
heterogeneity. This chapter starts by presenting data on the aerial and
vertical extent of the aquifer material. The second portion defines the
hydraulic properties of the aquifer.

AQUIFER GEOMETRY

The Quaternary deposits shown on the geologic map (Figure 2.6) indicate
the surface extent of the Missoula Aquifer in the study area. The aquifer
covers approximately 35 mi^ . The vertical extent of the aquifer can be
approximated from the hundreds of driller's reports and well logs on file at
the Missoula office of the Montana Department of Natural Resources and
Conservation. Summaries of these well logs are presented in Appendix 3A.
Morgan (1986) and Clark (1986) attempted to delineate the aquifer base and the
internal stratigraphy of the aquifer from these records. Much of the
following description is taken from Morgan's and Clark's research.

Aquifer geometry and stratigraphy were evaluated by interpretation of
geologic logs recorded on driller's reports and from the collection and
description of cuttings taken from nine wells drilled during the course of
this project. Field inspection of excavations and road cuts in which the
unsaturated portion of the aquifer was exposed were also examined.

Interpretation of the aquifer base proved more difficult than
anticipated. Less than 20 wells were inferred to fully penetrate the aquifer.
Most of these were Mountain Water Company wells, which are concentrated in the
eastern portion of the valley.

Two criteria were used to determine the base of the aquifer. The first
is a downward change, seen in the driller's geologic log, from a coarse sand
and gravel sequence to a sequence dominated by silts and clays. This
transition presumably separates coarse grained sediments which yield water
from the underlying fine grained Tertiary sediments which do not yield
significant amounts of water.

The second criterion is based on well design and yield data recorded in
driller's reports. Total depths and perforated intervals of wells were used
to identify portions of the aquifer with higher transmissivity. These data
were then used in conjunction with yield and drawdown information reported by
drillers. Domestic wells penetrating the Missoula Aquifer produce 10 to 100

gpm with only a few feet of drawdown. In contrast, wells finished in Tertiary
sediments produce only a few gallons per minute with tens of feet of drawdown.

Seismic refraction work conducted by Clark (1986) was unsuccessful in
defining the base of the aquifer, which was assumed to be 150 to 200 ft below
land surface. Apparently the acoustic impedance between the sand and gravel
deposit and the underlying Renova equivalent sediments is too low to be
recognized with the equipment used (Bison Signal Enhancement Seismograph Model
1570B), and/or the interface is deeper than what the equipment can record.

Figure 3.1 presents data from driller's reports for three wells. Two of
the wells penetrate the aquifer in the Missoula Valley. A contrasting third
well is from the Nlnemile area and is finished in Tertiary sediments similar
to those found on the flanks of the Missoula Valley and beneath the Quaternary
sand and gravel of the valley floor. . '

Figure 3.2 presents the interpreted depth from land surface to the base
of the aquifer. Although data are insufficient to construct an isopach map,
cross sections could be constructed from well data to indicate the general
position of the aquifer base (Figures 3.3 to 3.7). Well data used for these
cross sections are summarized in Appendix 3A. Based on the cross sections,
the Missoula Aquifer averages between 110 and 140 ft in thickness but ranges
from 92 to 237 ft thick. These thickness values include the unsaturated
portion of the aquifer.

The saturated thickness of the aquifer ranges from 50 to 120 ft in the
most of the valley. The thinner sections occur in the central and northern
portion of the valley. Depth to water is typically 50 to 70 ft in the central
portion of the basin and at its northern contact with the Tertiary sediments.
The water table is closest to the surface (10 to 30 ft) adjacent to streams
and in the southwestern portion of the basin.

The Missoula Aquifer is thickest (over 200 ft) at the merger of the Grant
Creek Valley and the main Missoula Valley. In the spring when. Grant Creek is
actively recharging the aquifer, the saturated portion of the aquifer near the
mouth of the Grant Creek Valley is 130 to 150 ft thick with one site recorded
as 197 ft (Pottinger, 1988).

AQUIFER STRATIGRAPHY

Morgan (1986) developed a composite picture of the stratigraphy of the
Missoula Aquifer from his extensive review of driller's reports and
construction of cross sections (Figures 3.3 to 3.7). The stratigraphy noted
by Morgan is based on his interpretation of often sketchy driller's logs and
has not been confirmed by a detailed drilling program. He described four
stratigraphic units. The lowest unit correlates with the Tertiary Sediment
Hydrostratigraphic Unit and the other three are interpreted as parts of the
Missoula Aquifer Hydrostratigraphic Unit (Table 2.1), These upper three units
are renumbered and described below (Table 3.1).

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TABLE 3.1
Stratigraphic Zones of the Missoula Aquifer

Unit One: Interbedded boulders, cobbles and gravel with sand,

silt and some clay. Thickness from 10 to 30 ft. Found
at land surface.

Unit Two: Tan to yellow, silty sandy clay with local layers of

coarse sand and gravel. Thickness averages 40 ft in the
center of the basin (Figure 3.5) to 130 ft in the
northwest section of the study area (Figure 3.7).
Absent in places.

Unit Three: Interbedded gravel, sand, silt and clay. Unit seems to
be coarser at the bottom. Thickness varies from 50 to
100 ft. May include part of the Sixmile Creek Formation,

Unit One forms the surface of much of the valley floor. It is composed
of 10 to 30 ft of large boulders, cobbles, sand and silt and lacks the
abundant fine sediments of Unit Two (Figures 3.4 to 3.7). The large cobbles
and boulders are characteristic and are difficult to drill through. Unit One
was probably deposited by large aggrading river systems fed by glacial
meltwaters during the last stages of the Pleistocene.

To determine the size range and uniformity of aquifer material, sieve
analyses were conducted on four 50 lb sediment samples collected from the
upper unsaturated portion of the aquifer (Clark, 1986) (Table 3.2). Two
samples came from a vertical face of the excavation for the University of
Montana Stadium at the northeast corner of the campus and two from a gravel
bar in the Clark Fork River between the confluence of Rattlesnake Creek and
the University Walking Bridge. All four samples are very coarse and very
poorly sorted. Using the modified Wentworth grain-size scale, 90% of the
sediment is at least the size of coarse sand. Mean diameters are in the
coarse to very coarse pebble range. Values of the uniformity coefficient and
inclusive graphic standard deviation for all 4 samples indicate that each
sample is very poorly sorted (Folk, 1980).

Saturation of Unit One generally occurs only beneath and adjacent to
streams and in the southwestern corner of the aquifer. The rest of this unit
is above the saturated zone and therefore does not yield water. The shallow
ground water found in saturated portions of Unit One is more likely to be
affected by contaminants infiltrating from the surface than water found in
deeper units.

TABLE 3.2

Grain Size Data from Sieve Analyses for

Unit One, Missoula Aquifer

Sample 1 Sample 2 Sample 3 Sample 4

Location

excavation for

gravel

bar

stadium

Clark

Fork

River

Depth of

5-15

15-20

0-0.5

.5-1.0

sample (ft)

Effective

0.50

0.55

4.0

0.55

diameter (mm)

Mean diameter

19.8

53.5

79.7

60.3

(mm)

Uniformity

104

coefficient

Inclusive

2.31

2.92

3.18

2.74

graphic

standard

deviation

Unit Two generally consists of a yellow silty sandy clay with interbedded
gravel and sand lenses. Well logs do not contain sufficient information to
determine detailed stratigraphy, but drillers report a predominance of tan
clay, sand and gravel. Drillers note the existence of clay in this unit from
the return of cloudy tan to pink water with the drill cuttings. However,
cuttings of sand and gravel are also returned continuously during drilling of
some wells. Based on its lithology and the presence of the tan clay. Unit Two
is probably related to deposition of Pleistocene Glacial Lake Missoula
sediments which outcrop in the northwestern part of the study area (Figure
3.4). In fact, finer portions of the drill cuttings look very similar to
Glacial Lake Missoula sediments. During the repeated filling and draining of
Glacial Lake Missoula, a complex sequence of alluvial fans, stream channels
and deltas interf ingered with lake sediments on the flanks and floor of the
valley. Over 100 ft of fine grained lake sediments overlie gravel in the
western portion of the valley near Huson and appear to interfinger with the
western edge of the Missoula Aquifer (Figure 3.4). Unit Two is discontinuous
in some areas of the aquifer and may have been replaced by coarser fluvial
deposits (Figure 3.4 and 3.7). Few wells are developed in Unit Two because
Unit Three has a higher transmissivity .

Unit Three is dominated by coarse grained sediments, especially at the
base of the unit. Tan fine grained sediments are interlayered with sand and
gravel. The ratio of fine to coarse material must be low because wells
pumping over 3,000 gpm are developed in this unit. Unit Three probably
represents channel lag, point bar and floodplain deposits from a large fluvial
network which developed over time. Morphologically similar deposits are found
in the present Clark Fork River channel and floodplain. This large fluvial
could have developed either in the Pleistocene or late Tertiary.

Grimestad (1977) observed similar stratigraphy west of the study area at
the Stone Container Corporation mill near Frenchtown (Figure 2.1). He
described a 25 to 35 ft thick, sequence of sand and gravel at the surface; an
underlying 60 to 125 ft thick sequence of mixtures and lenses of silt, sand,
clay and gravel; and a basal 20 to 50 ft thick zone of sand and gravel resting
on bedrock. This sequence of finer material sandwiched between coarser units
correlates well with the three Missoula Aquifer units described previously.
The thickness and pronounced finer nature of Grimestad's middle layer
(compared to Unit Two) may be a result of either being farther from the
northeast portion of the valley where high energy streams enter, or because it
was lower in elevation and subject to longer periods of lake related
deposition.

In summary, the Missoula Aquifer appears to be composed of three
lithologic units which are identifiable throughout a large part of the
aquifer. All contain sand and gravel but have other features which determine
their hydrologic character. The top unit is bouldery but generally above the
saturated zone. The fine grained component of the middle zone appears to have
reduce water transmitting capacity. When it is present above this productive
basal zone, it may protect portions of the valley's water supply from quick
downward movement of contaminants through Unit One. The basal unit is very
coarse and currently yields large quantities of water to the many wells
developed in it.

HYDROLOGIC PROPERTIES

Important hydrologic properties of an aquifer include porosity, specific
yield or storage, hydraulic conductivity and transmissivity . Characterization
of these properties in the Missoula Aquifer was accomplished by reviewing the
literature, interpreting driller's reports and conducting grain size analyses,
perraeameter experiments, and aquifer pumping tests. Clark (1986) describes
the basic methods used to interpret aquifer properties and the following
material has been largely taken from his work. Additional results of Morgan
(1986) are included in the discussion of specific capacities.

All hydrologic analyses done in this study were based on the major
assumption that the aquifer is unconfined, isotropic and homogeneous. This
assumption was made to simplify calculations and to allow the maximum
interpretation of data. The description of the aquifer stratigraphy presented
above indicates that this simplifying assumption does not properly represent
the Missoula Aquifer. However, there is currently insufficient information
available to analyze the aquifer using a more accurate model. Even
considering the heterogeneities of the Missoula Aquifer, aquifer testing and
interpretation is believed to result in generally conservative estimates of
aquifer properties.

Four 50 gallon samples of aquifer material from Unit One were evaluated
by Clark (1986) to determine values for specific yield, porosity and hydraulic
conductivity. The hydraulic conductivities of the samples were also estimated
from the sieve analysis data using techniques developed by Slichter and
Terzaghi described in Fraser (1938). Aquifer properties derived from these

methods were considered estimates applicable to Unit One and, assuming a
similar depositional environment, Unit Three.

Aquifer properties for Units Two and Three were determined by analysis of
aquifer testing data. In the first method used, drawdown and pumping
information recorded on driller's reports were analyzed to calculate
transmissivity using a computer program (PTRAN) (Appendix 3B) . Clark (1986)
describes the assumptions used to develop the program. Several problems were
encountered using this technique. First, domestic wells posed a challenge as
they are typically finished as open ended casings and do not penetrate the
full aquifer thickness. In an attempt to use the maximum available data, an
aquifer penetration value of one foot was used for open ended wells. For most
domestic wells, aquifer thickness was interpreted as the length of continuous
saturated coarse sand and gravel extending upwards from the bottom of the well
casing. For large water company wells, the saturated thickness was assumed to
be the thickness of Unit Three or, if Unit Two was absent, the thickness of
Unit Three plus the saturated portion of Unit One. Second, inaccuracies in
measurements by drillers directly affect calculated transmissivity values.
For instance, pumping times are often rounded to the nearest hour, drawdown in
wells pumped by airlifting is difficult to determine and variations in
discharge may go undetected. These problems are more common for analysis of
data from smaller diameter wells drilled by rotary rigs than with the few
dozen larger diameter wells drilled by cable tool.

Aquifer testing on well MWC34 was the second method used to calculate
transmissivity for Unit Three (Plate 1). The tests also provided values of
horizontal and vertical hydraulic conductivity and specific yield. Clark
(1986) describes the details of the aquifer testing.

RESULTS OF AQUIFER PARAMETER DETERMIIIATION

Results of aquifer parameter determination from the combined laboratory
and field tests and from analyses of driller's reports are summarized in this
section. Results are organized by hydraulic units and summarized in Table
3.3.

TABLE 3.3
Estimates of Aquifer Properties

Property

Unit One

Unit Two

Unit Three

Porosity

0.20

Specific Yield

0.12

0.10

Thickness (ft)

10-30

50-150

Hydraulic Conductivity

10,300

200

10,300-

(gpd/ft2)

25,500

Vertical Hydraulic

970-2,100

Conductivity (gpd/ft^)

Transmissivity (gpd/ft)

103,000-

8,000

750,000-

310,000

1,710,000

Laboratory determinations of hydrologic properties of Unit One revealed
average values of 0.20 for porosity, 0.12 for specific yield, 0.08 for
specific retention, and 10,370 gpd/ft^ for hydraulic conductivity.

The sieve analyses of Unit One samples yielded hydraulic conductivity
values which vary over 2 orders of magnitude (Clark, 1986). The large
differences in calculated values are caused by the four to eight fold
variation in the mean and effective grain diameters of samples. An average
hydraulic conductivity value for the four samples computed with both methods
is 2,953,710 gpd/ft2.

Grimestad (1977) reported a hydraulic conductivity value of 10,250
gpd/ft^ and a specific yield of 0.20 to 0.35 for the surficial gravel which
corresponds to Unit One.

Morgan (1986) estimated hydraulic properties of Unit Two from driller's
specific capacity data from a small number of wells finished in what he
interpreted as the finer middle unit. Average specific capacity was 7 gpm/ft.
Using estimating techniques described by Driscoll (1986) which convert
specific capacity to transmissivity , Morgan calculated a representative
transmissivity for the unit of about 10,500 gpd/ft. Based on an average
thickness of 40 ft, the hydraulic conductivity is approximately 260 gpd/ft^.
Grimestad (1977) reported a hydraulic conductivity of 150 gpd/ft^ for his
middle unit.

Unit Three hydraulic properties were obtained from analysis of drillers's
reports and aquifer tests. Estimates by Clark (1986) of transmissivity
generated from specific capacity data using the computer program PTRAN are
presented in Appendix 3C. Transmissivity values computed from specific
capacity data range from 52 to 4,149,000 gpd/ft for domestic wells and average
365,600 gpd/ft. Values for municipal wells range from 48,000 to 9,752,000
gpd/ft and average 1,710,000 gpd/ft. An average value of transmissivity for
both municipal and domestic wells is 750,000 gpd/ft. Hydraulic conductivity
values for municipal wells range from 520 to 113,400 gpd/ft^ and average
25,500 gpd/ft^. Values for domestic wells range from 1 to 27,700 gpd/ft^ and
average 4,100 gpd/ft^. The average value for both municipal and domestic
wells is 10,300 gpd/ft^.

Morgan (1986) reported specific capacity data for over 50 wells completed
in Unit Three and found an average specific capacity value of 240 gpm/ft for
these wells. Estimation of hydraulic conductivity and transmissivity from
these data using Driscoll's (1986) method yields a range of hydraulic
conductivity values between 3,600 and 7,200 and a transmissivity of 360,000
gpd/ft (Driscoll, 1986).

Results of the two aquifer tests conducted by Clark (1986) provide
transmissivity values for Unit Three which vary considerably and range from
606,000 to 5,856,000 gpd/ft (Table 3.3). Specific yield values ranged from
0.03 to 0.07. Depending on the assumptions used, horizontal hydraulic
conductivity ranged from 6,400 gpd/ft^ to 62,300 gpd/ft^. Values of vertical
hydraulic conductivity ranged from 970 gpd/ft^ to 2,100 gpd/ft^. The tests

were not conducted under ideal conditions as Mountain Water Company's pumping
schedule for MWC34 could not be altered to benefit the tests.

The distribution of hydraulic conductivity and transmissivity values
calculated from specific capacity data by Clark (1986), Meyer (1985) and
Pottinger (1988) are presented in Figures 3.8 and 3.9. One pattern emerges in
the western portion of the study area where values of transmissivity and
hydraulic conductivity are lower (Clark, 1986, and Meyer, 1985). However, the
work (Meyer, 1985) which generated these lower values is poorly documented.
The distribution of hydraulic conductivity and transmissivity data in the
Missoula Aquifer also appears to reflect an area of higher hydraulic
conductivity and transmissivity values near the mouth of Hellgate Canyon and
adjacent area south and southwest of the river.

Values for aquifer properties of Unit Three generated for this report are
similar to values reported by previous workers. Using data from specific
capacity tests, McMurtrey and others (1965) determined transmissivity values
which varied from 17,800 to 1,000,000 gpd/ft. From an aquifer test in the
north central part of the study area, these workers also calculated a
transmissivity value of 620,000 gpd/ft. An aquifer test by Geldon (1979)
provided a transmissivity value of 699,927 gpd/ft. Hydrometrics (1984)
attempted an aquifer test on MWC34, the same well tested by Clark (1986), and
concluded that transmissivity ranged from 250,000 to 1,000,000 gpd/ft, in good
agreement with Clark (1986). From 19 aquifer tests, Geldon (1979) reports an
average hydraulic conductivity value of 5,090 gpd/ft^. In the same test
series, he determined specific capacity values ranging from 3 to 3,000 gpm/ft.

Grimestad (1977) reported a hydraulic conductivity range of 3,400 to
16,660 gpd/ft^ for Unit Three in the central valley area. McMurtrey and
others (1965) reported that the transmissivity for two wells producing water
from the same unit ranges from 77,000 to 125,000 gpd/ft.

McMurtrey and others (1965) assumed a porosity value of 0.40 and a
specific yield of 0.10. From aquifer tests Geldon (1979) found time-dependent
specific yield values which ranged from 0.11 to 0.35.

SUMMARY OF HTDR0L06IC PROPERTIES

The vertical and horizontal heterogeneities in stratigraphy throughout
the aquifer contribute to the wide range in hydrologic values. It is
difficult to generalize about these properties. The most reliable values for
each property have been selected and are summarized in Table 3.3. The
rationale for selecting each value is explained below. Generally, it is
believed that aquifer properties derived from aquifer tests, permeameter
experiments and driller's log analyses are most accurate. Hydraulic
conductivity values calculated from sieve analyses were considered too high
and were rejected because the techniques are intended for sand and not coarse
sand and gravel.

The hydraulic properties assigned to Unit One are based on the
permeameter testing data presented by Clark (1986) and thickness data reported
by Morgan (1986). Transmissivity values were obtained by multiplication of

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hydraulic conductivity by thickness. For Unit Two specific capacity and
saturated thickness data came from Morgan (1986). Hydraulic conductivity
estimates were obtained by averaging interpretations of Morgan's data for Unit
Two and Grimestad's (1977) value for his intermediate unit.

In Unit Three porosity was derived from the permeameter work by Clark
(1986). Specific yield was estimated from the permeameter data, from aquifer
tests conducted by Clark (1986), and from McMurtrey and others (1965).
Aquifer thickness is presented as a range of values based generally on work by
Clark (1986), Morgan (1986) and Pottinger (1988). Hydraulic conductivity is
presented as a range. The low value corresponds to permeameter results
(Clark, 1986) and the higher value represents an average calculated from
Mountain Water Company well specific capacity data. Vertical hydraulic
conductivity values come from Clark's (1986) aquifer tests. The selected
transmissivity values were calculated from specific capacity data. The lower
value is the average transmissivity computed from all specific capacity data.
The higher value is the average from specific capacity data from just Mountain
Water Company wells. This range of transmissivity values is supported by
Clark's (1986) aquifer tests.

Single of single values for transmissivity and hydraulic conductivity for
Unit Three were selected based on data obtained from wells which were drilled
with cable tool methods, had perforated aquifer intervals with known aquifer
thickness, and whose pumping tests were at least four hours long.
Transmissivity values from municipal wells (MWC) best fit these criteria and
average 1,710,000 gpd/ft. Hydraulic conductivity values average 25,500
gpd/ft2.

The values given in Table 3.3 provide a general description of the
Missoula Aquifer and indicate that the aquifer is highly conductive. Unit Two
appears to be significantly lower in transmissive ability than Units One or
Three, although the actual range and spacial variation in Unit Two's hydraulic
properties throughout the study area are not well understood. Hydraulic
conductivity and transmissivity values appear extremely high for Unit Three.
Operation of large production wells in this unit have small drawdowns, which
support these high values. As an example, well MWC3A pumps at 7,000 gpm and
typically has less than eight feet of drawdown at the well. MWC23 and MWC35
are located away from the river and yield 250 and 2,500 gpm with one and six
feet of drawdown, respectively (Plate 1).

In an attempt to characterize the aquifer with just a few numbers, the
hydrologic properties were calculated for a hypothetical composite aquifer
composed of 10 ft of Unit One, 20 ft of Unit Two and 60 ft of Unit Three. The
weighted average value for hydraulic conductivity is 18,200 gpd/ft^ and
1,152,000 gpd/ft for transmissivity. Single values for porosity and specific
yield are 0.20 and 0.12, respectively.

CHAPTER 4
WATER TABLE FLUCTUATIONS

This chapter addresses three objectives: 1) describe the network of
observation wells used to measure changes in water table elevation and aquifer
storage during the study; 2) present and discuss the observed data; and 3)
evaluate these water level changes in a historical context.

HONITORIRG WELL HETHORR

The well monitoring network developed during this study is composed of
several different groups of wells. In the summer of 1985 the first 31
domestic wells were chosen and given the MV label. Criteria for selection of
these wells included access, potential for long term monitoring, availability
of a driller's log, and location in the flow system. Twenty-nine of these
wells were monitored monthly until summer 1987, Two wells were monitored
monthly until summer 1986. Well locations are presented on Plate 1 and in
Figure 4.1.

Additional water level data for the valley were obtained from a 48 well
network (labeled P) used by Pottinger (1988) during a detailed investigation
in the vicinity of Interstate 90 and Grant Creek (Plate 1) and 24 Mountain
Water Company (MWC) wells (Figure 4.1). Pottinger (1988) measured water
levels monthly from summer 1985 to summer 1986. Water level records for most
MWC wells begin in the mid 1970' s. Typically, measurements were recorded one
to three times each year until 1984 when monthly water level records were
kept. .Monthly measurements were made at all MWC wells throughout the duration
of this project.

The only well for which there is a water level record longer than the MWC
wells is MV31 (also MWC7) located in the parking lot of Montana Power
Company's Missoula office. This well was monitored monthly by the USGS from
September 1958 until December 1966; quarterly by the USGS until July 1981,
sporadically by the MWC and USGS from 1981 to 1984, and monthly by this
project starting in summer 1985. An electrical transducer system capable of
recording daily water level fluctuations was installed in July, 1986.

By December 1985, seven additional monitoring wells were constructed as
part of this project (MV34 to MV40) and were monitored monthly (Figure 4.2).
Two additional wells (MV41 and MV42) were constructed in summer 1986 as part
of the storm drain study (Wogsland, 1988). These nine wells were drilled with
a direct air rotary drilling rig equipped with a drill-through casing driver.
Ten inch surface casing was set to 20 ft below land surface and six inch steel
casing was advanced to approximately 10 ft below the water table. Drill
cuttings were collected at five foot intervals. The wells were perforated
with a down-hole perforator over a 10 to 20 ft interval approximately centered
at the water table. Five rounds of perforations were cut. Each perforation
was 0.10 ft long and 0.01 ft wide. The wells were grouted with a cement
slurry which was poured in from the top of the ten inch surface casing as this
ten inch casing was removed. To complete construction, a steel instrument

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shelter was welded to each well. The two wells completed in 1986 were
finished with flush coupled PVC casing and 10 ft of 60 slot PVC well screen.
The six inch steel casing was removed and the drill hole allowed to collapse
around the screen. Two to three feet of bentonite pellets Were installed
above the screened interval. Twenty feet of six inch diameter steel casing
was left in the top of the bore hole to protect the PVC casing and for
attaching an instrument housing for a Stevens Type F continuous recorder.
Well logs for each of the nine wells are presented in Appendix 4A.

All monthly water level data collected as part of this study were
measured using steel tape. Water levels collected by Mountain Water Company
were measured using either a tape or air line. Well inventory information for
wells analyzed as part of this study is found in Appendix 3A. All water level
data are tabulated in Appendix 4B.

Casing elevations in feet above mean sea level were surveyed by the
Missoula County Engineers Office. Elevations are accurate to 0.01 ft.

One objective of this project was to construct a long term water level
monitoring network for the Missoula Valley. Well MV31 and seven of the
project constructed monitoring wells were equipped with electrical transducer
systems in early summer 1986 (Figure A. 3). The use of transducers for long
terra monitoring of water level fluctuations Is new. Systems installed in the
Missoula Valley were modified from designs developed by the Northern Rocky
Research group of the U. S. Forest Service (Prellwitz and Babbit, 1984).

RESULTS OF HATER LEVEL MONITORING

Data from the following wells illustrate the water level trends observed
during the 1985 to 1987 study period:

1. MV34, MV35 and MV6 , near the Clark Fork River;

2. MV31 and MV26, in the central portion of the area east of the Clark

Fork River;

3. MVIO, west of the Clark Fork River;

4. MV39 and MV20, near the Clark Fork and Bitterroot Rivers in the

western portion of the study area;

5. MV2 and MV36, at the mouth of Rattlesnake Creek; and

6. P31, at the mouth of Grant Creek.

Figures 4.4 to 4.10 show hydrographs for each of these wells. Monthly water
level data for all wells included in the monitoring well network are presented
in Appendix 4B.

In addition to measurements of monthly water levels, daily water levels
were measured at eight wells equipped with electrical transducers. The
results of the transducer operation and actual monthly water levels are
presented in Figures 4.11 to 4.14. Though battery problems affected data
collection, transducer results are encouraging. The daily records generated
from the transducer equipped wells support all trends observed from the
monthly hydrograph data. Records for MV31 and MV36 show that transducer
systems can be used to accurately record water level fluctuations (Figures

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3.147 -

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3.146 -

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3.140 -

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Dec- 83

Figure A. 4:

Jul- 86

Jon-B7

Aug-B7

Dot*

Hydrographs of data from wells MV34 (top) and MV35 (bottom)

MV06

3.14B

3.147 -

3.148 -
3.145 -

^ 3.144 -

r 3.143 -

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3.138 -

3.137 -

3.13B -

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Figure 4.5: Hydrograph of data from well MV6.

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MV31 — Montana Power

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3.129 -

3.128 -

5.127 -

3.126 -

3.125 -

Q 3.124-

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MV26 — South Avenue

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Jun— 88

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Figure 4.6: Hydrographs of data from wells MV31 (top) and MV26 (bottom)

MV10

3.145

3.144

3.143 -

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3.141 -

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3,137 -
3.136 -
3.135 -
3.134

3.133

Feb-B5

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Figure A. 7: Hydrograph of data from well MVIO

Buckhouse - MV39

3.123

3.123 -
3.122
3.122
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3.119 -
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3.1102

3.1100 -
3l1098 -
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3l1094 -
5.1092 -
3.1090 -
3.1 0B8 -
3.1086 -
3.1084 -
3.1082 -
3.1080 -
3.1078 -
3.1076 -
3.1074 -
3.1072 -
3.1070 -
3.1068 -I

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MV20, 4412 Spurgin Rd.

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Jun— BS

Jun— B7

Figure 4.8: Hydrographs of data from wells MV39 (top) and MV20 (bottom).

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3.232

3.231 -
3.230 -
3.229 -
3.228 -

3.227 -

3.228 -
3.225 -
3.224 -
3.223 -
3.222 -
3.221 -
3.220 -
3.219 -
3.218 -

3.217 -

MV2 - 1250 Monroe

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Greggory Park - MV36

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Figure 4.9 Hydrographs of data from wells MV2 (top) and MV36 (bottom)

Well #P31

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S.162

MADISON ST. BRIDGE - MV34

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3.152 -

5.15-
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3.144 -
3.142 -

3.14 -
3.138 -
5.136 -

5.134

Mar-B6

MV35. McCormick Park

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Figure 4.11: Hydrographs of transducer data from wells MV34 (top) and MV35
(bottom) .

8.142
3.141 -
5.140 -
8.139 -
8.138 -

8.157 -

5.158 -
8.138 -
8.134 -
8.138
8.132 H

8.131

MV-37, Reserve Street

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-86

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Jul-B8 0«f-88

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Jur>— 86

MV31, Montana Power

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Figure 4.12: Hydrographs of transducer data from wells MV37 (top) and MV31
(bottom) .

3.12B

3.127 -

5.120

K '■■

3.125

3.124

3.123

3.122 -

Jul-86

MV40. State Lands

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Jul-B6

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DATE

Figure 4.13: Hydrographs of transducer data from wells MV40 (top) and MV39
(bottom).

3.106

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3.101 -

MV38, SOUTH AVENUE

W«il Hydrograph

Oct-86

Jan— 87

T 1

May-B7

Au«-87

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3.200 -r

3.199 -

3.198 -

3.197 -

I 3.196 -
3.195 -

3.194 -

3.193

M«-86

MV36, Greggary Park

Well Hydroyoph

TRANSDUCER

DATE

^ Sl««l top*

Figure 4.14: Hydrographs of transducer data from wells MV38 (top) and MV36
(bottom) .

4,12 and 4.14), A number of the sites sustained battery failure In the
winter. At other sites, minor operational problems were not recognized
because of the field staff's lack of experience. This resulted in partial
loss of daily records at the remaining stations. Transducers at MV40, MV34 ,
MV35 and MV37 appeared to work well before power loss. Stations MV38 and MV39
required modification to obtain acceptable records.

Five significant trends are shown in Figures 4,4 to 4,14. The first is
an annual water level rise which occurs between March and June and then a
general decline until the following February or March, Table 4,1 summarizes
the measured water level rises from the winter low to the spring peak.
Transducer data were used in conjunction with monthly measurements to derive
1986 changes at MV34 and MV36 , The spring rises were 6,5 to 12 ft in 1986 and
7.5 to 10 ft in 1987 at wells located immediately adjacent to the Clark ForK
River (MV34, MV35 and MV6). MV31 is located further away from the Clark Fork
River and showed spring water level rises of seven feet in 1986 and eight feet
in 1987. MV26 increased about six feet during the spring of both years. The
water table north of the Clark Fork River rose about eight feet at MVIO during
both 1986 and 1987. Water levels at MV6 and MVIO were also collected prior to
spring peak in 1985. These data show a decline of about eight feet from the
peak to March. Spring water table rises at MV20 were about two feet each year
and at MV39, two and one half to three feet. At the mouth of Rattlesnake
Creek spring rises were at least four feet in 1986 and two feet in 1987 at
MV36. Well MV2, located about a quarter of a mile up the creek, showed a
three feet rise in 1986 and a 13 ft rise in 1987, This well has a peak water
table position in the spring, and the lows occurred in late winter in 1986 and
late fall in 1987, It is unclear what caused the drop and recovery of water
levels in the fall of 1986, Possibly this water level reflects pumping of
adjacent wells. Though only partial data are available to define the 1985
peak, the water table at P31, located at the mouth of Grant Creek, declined
approximately 17 ft from the high measured in 1985,

TABLE 4,1
Net Water Table Rises from Winter to Spring Peak

(feet)

Well 1986 1987

MV34 12* 10

MV35
MV6
MV31
MV26
MVIO
MV20
MV39
MV2
MV36
(*Transducer data used to define peak)

6,5

7.5

8.5

2.5

The second trend is defined by a decrease in the elevation of the spring
peak water level at all wells in 1986 than in 1987. Minimum water table
elevations were also lower in 1987 than 1986. Water table peaks at MV6 and
MVIO were less than one foot higher in 1985 than 1986. The third trend is
observed in seasonal water table fluctuations; the fluctuations decrease with
increasing southward from the Clark Fork River, Fourth, wells located in the
Missoula Aquifer near the mouths of Grant Creek and possibly Rattlesnake Creek
show the largest annual fluctuations in water table elevation (Figures 4,9 and
4.10). Finally, water levels in wells located near the Bitterroot River are
correlated with flow in the Bitterroot River and indicate that the ground
water and river systems are linked. Maximum water level rises are on the
order of three feet for wells located adjacent to river (Figure 4,8).

These trends probably reflect seasonal variation in recharge to the
aquifer and increased withdrawal for public water supply. The maximum water
table values observed between May and July correlate with the Clark Fork River
stage as shown in Figures 4.15 and 4.16. Losses measured by Clark (1986) in
stream discharge between the University Walking Bridge and the Reserve Street
Bridge indicate that the river recharges the aquifer. Other evidence that
recharge occurs from the Clark Fork River includes potentiometric maps which
show that ground water flows to the north and south away from the river
(Chapter 5) , river stage elevations which are higher than the adjacent water
table (Figures 4.15 and 4.16), and water level fluctuations at wells close to
the river channel which are greater than those at wells farther from the river
(Figure 4.17).

The water table declines naturally in response to seasonal reductions in
Clark Fork River discharge and corresponding decreases in ground water
recharge and to continued natural discharge from the system to the Bitterroot
River and lower reaches of the Clark Fork River (Figure 4.18). The elevation
of the water table at well MV39 is higher than the stage of the Bitterroot
River, which indicates flow is maintained to the river from the aquifer. In
addition to natural discharge from the system, ground water is withdrawn from
individual, industrial and municipal wells. Although withdrawals vary
seasonally, most total annual withdrawals have remained constant since the
late 1970's. The one exception is the withdrawal of water from the aquifer by
Mountain Water Company. Figure 4.19 presents the total monthly ground water
withdrawal by the Mountain Water Company well network (Appendix 4C). Until
1983, just over 4 billion gallons of water were used each year, with the
largest production occurring in the summer. After the abandonment of the
Rattlesnake surface water supply in summer 1983, annual ground water
withdrawal has been over 9.7 billion gallons. Peak monthly demands since 1983
have exceeded 1.2 billion gallons.

HISTORICAL WATER TABLE TREHDS

The water level data collected and compiled as part of this study
represent the most extensive documentation of water table trends in the valley
to date. However, in order to evaluate the long term trends in water table
elevations, it is necessary to assess records collected over as many years as
possible. The records for well MV31 (MWC7) provides water level data from
1958 to the present. This well is not pumped.

S.157

3.156 -
3.155
3.154-
3.153-
i 3.152 H

I i 3.15 H
^ 3.149 H
3.148-
3.147 -
3.146-
3.145 -
3.144

Madison St. Well - MV34

Stotfe Wotor Uv«| v*. 1bn«

7/83

11/88

3/86

7/86

11/86

3/87

7/87

3.1776

Clark Fork River ~ North Walking Bridge

11/8S

-I — »"
3/86

3/87

7/87

Figure 4.15:

Hydrographs of data from well MV34 (top) and the Clark Fork
River at the University Walking Bridge (bottom).

t C

3.142
5.141
3.14
3.139
3.1 38
3.137
3.136
3.13S
3.134
3.133
3.132
3.131

RESERVE ST. WELL - MV37

STATIC WWtR LEVEL VS. TIME

I t t I ■ 1 1— —J r-

T p— r-

I • 1 I I • »~

7/83 11/83 3/88 7/88

UONTTORINC MOHmS

11/86

3/87

CLARK FORK RIVER - RESERVE ST. N.BRIDGE

STAGE vs. TIME

3.1455

3.145 -

i1445 -

3.144 -

1^^

3.142

3.1415

—I r— I 1 1 r-

7/B5 1 1/85

» < 1 1 I I

-I 1 t

3/86 7/86

UOWrORINC MONTHS

11/86

3/87

Figure A. 16: Hydrographs of data from well MV37 (top) and the Clark Fork
River at the Reserve Street Bridge (bottom).

CLARK FORK RIVER - NORTH WALK BRIDGE

STAGE vs. TIME

«5 5.1 782
*^| 5.1760
!i g i175B
^£5.1756

7/B5 11/B5 3/B6 7/B8

UONITORINO MONTHS

11/B8

3/87

7/B7

9.16

S.15-

5.14-

3.13-

5.12

3,11 -

Well Hydrographs

3.1 -f

7/B5

11/85

3/88

7/88

11/88

3/87

7/87

amv34

rnv6

mv13

mv16

mv18

mv20

Figure 4.17: Hydrographs of data from the Clark Fork River (top) and wells
located progressively further from the river (bottom).

BUCKHOUSE BRIDGE WELL - MV39

STATIC WATKR LEVEL VS TME

i123

5.1225-

5.122-

5.1215-

t c

■ "

5.121 -
5.1205 -

5.12-

11195-

5.119 -

5.1 1B5 -

7/BS 11/85 3/B6 7/88

uoNrroRiNC months

11/86

3/87

BITTERROOT RIVER - BUCKHOUSE BRIDGE

STAGE vs. TWE

5.1195

5.118

5.1155

-I • I 1 r— — t 1 I I r— I 1 1 I 1 I I

11/B5 3/88 7/88 11/88

UONfTCRINC MONTHS
Figure 4.18: Hydrographs of data from well MV39 (top) and the Bitterroot
River at Backhouse Bridge (bottom).

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The water level trends observed in Figure 4.20 show a general fluctuation
between about 3,146 ft to 3,132 ft from 1958 through 1980. From 1980 to 1985
water level data were collected at times which would not reflect periods of
highest and lowest water levels so trends are difficult to distinguish with
these data. Since 1984, monthly water level data show maximum and minimum
values fluctuating between about 3,140 and 3,130 ft.

Review of historical water level records from MWC wells shows trends
similar to those of MV31. Figures 4.21 and 4.22 present typical hydrographs
for Mountain Water Company wells. The water level trends for wells MWC34 and
MWC30, which are located adjacent to the Clark Fork River, fluctuate about ten
feet seasonally. They both show an apparent stepwise lowering in water levels
in late 1983. A similar, but less pronounced, trend is seen in wells MWC19,
located north of the river, and MWC20, located to the south. All of these
wells are active pumping wells in the MWC network.

Analysis of the long term water level trends in the valley provides an
indication of how the aquifer is responding to climatic influences and pumping
stress. The hydrographs presented to illustrate the longer terra trends show
what appears to be a general lowering of the valley water table since 1983.
This trend can be seen by looking at the lower elevations of yearly water
table raaxirauras and minimums between 1983 and 1987. This record is too short
to adequately assess whether a long term trend of net annual decline is
actually occurring. This apparent lowering of the water table can be
attributed to the Increased withdrawal which Mountain Water Company started in
1983 (Figure 4.19) and below average Clark Fork River annual and peak
discharges which also began in 1983 and continued through 1987 (Figure 4.23).
It is not possible with just water level trend data to separate entirely the
individual affect of each of these factors. The source or sources of water
level decline will be examined using a mass balance approach in Chapter 5.

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3.151 -

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Oct-80 Jul-83 Mar-86 D«o-8B

Dot*

MWC30

Z o

DATE

Figure 4.21 Hydrographs of data from wells MWC3A (top) and MWC30
(bottom).

MWC19

3.151

3.1 38

Apr-75

MWC20

— I —

Jon— 78

Oct- 80

"1 1

Jul-83

1 r

Mar-86

D«e-B8

DATE

Figure 4.22:

Hydrographs of data from wells MWC19 (top) and MWC20
(bottom).

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CHAPTERS
GRODHD WATER FLOW: SOURCES OF RECHARGE AND DISCHARGE

Chapter 4 described the monitoring well network established during this
project and the variations in water table elevations which occurred in the
Missoula Aquifer. This chapter describes aquifer recharge and discharge,
general ground water flow paths and seasonal variations in the flow field.

GROUND WATER FLOW SYSTEM

Monthly water table elevations collected at over 50 wells were plotted
and contoured to produce maps showing the configuration of the top of the
saturated zone in the aquifer. Three of these potentiometric maps are
presented (Figures 5.1 to 5.3) as examples of water level fluctuations and
variations in flow paths during 1986. The months selected were:

1. March, when the water table was at its lowest for the year;

2. June, when water levels were highest; and

3. October, as water levels were declining.

The same general pattern of ground water flow is maintained during most
of the year. North of the Clark Fork River, ground water moves away from the
channel and away from the northern aquifer boundary, where ground water from
the Tertiary sediments and the alluvium in the Grant Creek and Rattlesnake
Creek Valleys recharges the aquifer. The net result is ground water migration
parallel to the river channel and final discharge to the Clark Fork River
north of the river's confluence with the Bitterroot River. South of the river
ground water flows southwest towards the Bitterroot River and towards the
confluence of the Clark Fork and Bitterroot Rivers.

By comparing the water table elevations adjacent to the Clark Fork River
with a river channel profile derived from USGS topographic maps. It is evident
that the stream is influent over a reach downstream of Hellgate Canyon (Figure
5.4). The length of the influent reach varies seasonally, from about four
miles during June and July when the water table is at its highest and over six
miles during the winter and spring. Figure 5.4 also shows that the largest
differences between the elevations of the river channel and water table occur
in the first three to four miles below Hellgate Canyon. Water level trends
discussed in Chapter 4 and water quality data presented in Chapter 6 also
indicate that the Clark Fork River recharges the aquifer.

Variations in the rate at which the Clark Fork River and other sources
recharge the aquifer are reflected in the buildup of the water table in the
late spring and early summer, and its subsequent decline from mid summer
through late winter. A decrease in recharge from late June to the following
March caused the 3,135 ft equipotential line to migrate over two miles to the
northeast (Figure 5.5). The almost identical position of the this
equipotential line southwest of the first two miles of river channel from
December through March probably implies that aquifer recharge and discharge
relationships have stabilized. During the same period, additional migration

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of the 3,135 ft equipotential line Is observed north of the river in the
vicinity of the Reserve Street Bridge. This trend is most likely controlled
by a decline in recharge from the influent Grant Creek and a continuing
seasonal reduction in recharge from the Tertiary sediments to the north.

AQUIFER RECHAR6B AND DISCHARGE

Water recharges the Missoula Aquifer from a number of sources:

1. Influent reaches of the Clark Fork River;

2. Direct precipitation on the aquifer;

3. Discharge from the adjacent Tertiary Sediment and Bedrock

Hydrostratigraphic Units;

4. Storm water runoff;

5. Septic systems; and

6. Leakage from irrigation ditches.

Leakage from the Clark Fork River in the valley is the major source of
recharge to the Missoula Aquifer. Clark (1986) attempted to quantify the
recharge from a three mile reach of the river by measuring the loss in
streamflow between the University Walking Bridge (about 0.5 mi from the mouth
of the Hellgate Canyon) and the Reserve Street Bridge, After accounting for
irrigation diversions in this reach and inflow from Rattlesnake Creek, he
concluded that about 14% of the flow measured at the Walking Bridge, or
324,700 acft/yr, leaked through the streambed and recharged the aquifer
(Figure 5.6). Clark's (1986) calculations were further refined by
incorporating the variations in the length of the influent reach of the river
and by estimating monthly instead of annual seepage losses (Figure 5.7), The
revised estimate of leakage from the river is 412,700 acft/yr. Leakage
calculations are explained in Appendix 5A.

The role of direct precipitation on the unconfined aquifer has not been
quantified. If precipitation contributes to aquifer recharge at all, it
probably occurs only in the spring associated with snow melt and spring
rainfall. By July, all rainwater not entering the Missoula storm water
systems is probably evapotransplred. From November to March, the ground is
frozen and recharge from the surface does not occur.

Spring precipitation and snowmelt compose the principal recharge to
bedrock and Tertiary sediments which underlie surrounding highlands. Water
from these hydrostratigraphic units then recharges the Missoula Aquifer
through lateral inflow. Equipotential lines indicate a source of recharge
from the mountainous terrain north of the valley (Figures 5.1 to 5.3). These
sediments do yield small volumes of water to wells; equipotential surfaces in
these sediments are higher than in the Missoula Aquifer.

The annual ground water discharge from the hills on the aquifer's
northern boundary was estimated by assuming an aquifer thickness of 75 ft, a
hydraulic conductivity of 100 ft/d and a hydraulic gradient of 0.006 for this
6.75 mi boundary (Pottinger, 1988). Calculated discharge is 13,400 acft/yr.

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Some minor recharge may also come from the hills on the east and southeast of
the valley and from underlying formations. These sources were not quantified.

Ground water flow from other small stream valleys is an additional source
of recharge in the northern portion of the aquifer. Sendler (1986) estimated
that 350 acft/yr of ground water discharges from the Rattlesnake Creek
alluvium to the Missoula Aquifer, du Breuil (1983) estimated ground water
recharge from the Grant Creek Valley at 4,900 acft/yr. Smaller streams, such
as Pattee Creek and Butler Creek, also are influent as they enter the Missoula
Valley. Their recharge rates were not quantified.

Storm water runoff in the Missoula area is channeled into over 2,000 dry
wells which allow water to percolate to the water table. Wogsland (1988)
estimated that 365 acft of water are injected annually.

Septic systems service all but the main Missoula metropolitan area. Over
5,700 individual residences are found in the valley with an additional 600 in
the Rattlesnake Creek Valley. Ver Hey (1987) measured daily loading of about
250 gpd per household. Based on her research, about 1,600 acft/yr of septic
wastes recharge the aquifer.

Geldon (1979) attempted to quantify the seepage from irrigation ditches
and irrigated areas for the immediate area around Missoula. Though values
have not been derived for the entire aquifer, his results provide an
approximation good to an order of magnitude. He estimated that irrigation
practices account for approximately 8,500 acft/yr of aquifer recharge.

In the Missoula area, losses from Mountain Water Company's distribution
system are estimated to be over 12,250 acft/yr, assuming a 50 percent loss of
the water produced. Table 5.1 summarizes the sources of recharge to the

Missoula Aquifer.

Water which discharges from the aquifer by evapotranspiration and as base
flow to streams has not been quantified. Evapotranspiration is probably an
important source of aquifer discharge in riparian areas along streams and from
subirrigated fields west of the confluence of the Bitterroot and Clark Fork
Rivers .

Ground water is pumped from wells owned by individuals, industrial users,
and two private water companies, Mountain Water Company and Clark Fork Water
Company (CFWC) . For the last two years MWC and CFWC have produced an average
of over nine billion gallons per year from the aquifer. About 45,000 people
live within MWC's service area and on the average use 550 gallons per person
per day (MCCHD, 1987). This daily use rate is high because of the 50 percent
water loss from the distribution system. The Clark Fork Water Company serves
2,329 people, who each use about 360 gallons daily. An estimated 4,700
families are served by single or multi-family wells outsideof the water
company service areas. Each family is assumed to use about 320 gpd (MCCHD,
1987). Total annual withdrawal from wells is estimated at 30,210 acft.

1,600

0.4

350

0.07

365

0.08

4,900

1.0

8,500

1.9

12,300

2.7

13,400

3.0

412,700

90.9

TABLE 5.1
Volume Estimates of Recharge to the Missoula Aquifer

Source Amount

acft/yr % of total

Septic systems
Rattlesnake Creek Valley
Storm Water*
Grant Creek**
Irrigation***
MWC line loss
Lateral inflow, north
Clark Fork River

TOTAL 454,115

(*Missoula area)
(**No other creeks quantified)
(***0nly in the immediate Missoula area)

The volume of natural discharge from the Missoula Aquifer was
approximated (assuming no net change in storage) by subtracting the estimated
annual recharge to the aquifer (Table 5.1) from the annual withdrawal from
wells (Table 5.2). The estimated annual discharge is 423,905 acft.

TABLE 5.2
Water Use from the Missoula Aquifer

Population Use Per Capita

Daily (gpd) Annual (acft/yr)

Mountain Water Company 44,755 550 27,570

Clark Fork Water Company 2,329 360 940

Households with 4,700 320* 1,700

private wells

(*gallons per household)

Total 30,210

In summary, aquifer recharge exceeds estimated ground water discharge and
withdrawal by 15 times. The leakage of water through the bed of the Clark
Fork River accounts for over 90% of the annual recharge. Therefore water
table declines noted in Chapter 4 most likely reflect recent climatic
conditions which have caused reductions in recharge rates from the river and
from Tertiary sediments. Water levels may decline locally in areas of heavy
pumping, particularly during periods of low recharge, but in general, current
levels of ground water withdrawal do not appear to be over drawing the
aquifer.

CHAPTER 6
AQUIFER HATER QUALITY

This chapter includes a discussion of sampling methodology, general
aquifer chemistry, results of an intensive sampling of the aquifer west of
Reserve Street and summaries of three ground water quality research efforts
conducted in conjunction with and partially support by this study.

METH(H>S

Twenty-six monitoring wells, six Mountain Water Company wells, and two
locations on both the Clark Fork and Bitterroot Rivers were sampled five times
on a quarterly basis between February 1986 and May 1987 (Figure 6.1). All
samples were analyzed for gross ionic chemistry by Dr. Juday (Department of
Chemistry, University of Montana). Energy Labs of Billings, Montana,
determined dissolved trace metal content of samples from lA of the 26 sites.
Standard sampling and analytical procedures were exercised. Analytical data
are presented in Appendix 6A and 6B.

The quality control program utilized duplicates, blanks, and standards.
Three sample duplicates and three blanks were added to each group of 26
samples delivered to the laboratories for gross ionic analysis. Each batch of
trace metal samples had one blank, one duplicate, and one trace metal standard
(prepared from National Bureau of Standard's SRM i!fl643b.)

The last quarterly sampling (spring 1987) included an expanded set of
monitoring wells. Sixty-eight domestic wells located west of Reserve Street
plus 28 sites in the quarterly monitoring network (Plate 1) were sampled for
gross ionic chemistry and coliform bacteria (Figure 6.2). Gross chemistry
analyses were performed by Dr. Juday. Coliform analyses were completed by the
Missoula City County Health Department. Sixty-six dissolved trace metal
analyses were done by Dr. Moore (Department of Geology, University of
Montana). Sampling followed standard procedures. A quality control program
similar to the one described above was used.

In June 1987, six ground water samples were collected and analyzed for an
EPA priority pollutant scan, which includes analyses for pesticides, PCB's,
and purgable, acid extractable and base neutral organic compounds. Standard
procedures were followed when sampling and analyses were performed using EPA
standard procedures by Lancaster Laboratories of Lancaster, Pennsylvania,
Blanks and duplicates were not included for quality control because of the
high cost of analysis and the limited sampling budget.

RESULTS: GEHERAL DESCRIPTION OF GROUND WATER QUALITY

The ground water in the Missoula Aquifer is a calcium bicarbonate type.
Near the Clark Fork River, the mineral content of ground water fluctuates
seasonally in response to changes in river water quality. Throughout the rest
of the aquifer, mineral content remains essentially constant at any one point
but increases in the direction of ground water flow. Detectable metal

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concentrations occur primarily in Clark Fork River water and in adjacent
wells. Stiff diagrams indicate the relative proportion of major ions in
ground water throughout the aquifer (Figure 6.3). Average and representative
water analyses are shown in Tables 6.1 and 6.2.

TABLE 6.1

Water Analyses of Representative Missoula Valley

Ground Water Samples Collected February 5, 1986

(mg/1)

Parameter

Up gradient

Down gradient

MV34

MV25

Bicarbonate

177.9

199.3

Chloride

2.46

5.55

Sulfate

19.3

22.1

N03 (as N)

0.41

1.13

Calcium

41.3

47.0

Magnesium

11.6

14.4

Sodium

5.9

7.4

Potassium

1.3

2.0

TDS

261.6

302.6

TABLE 6.2
Average Water Quality for All Water Analyses
(mg/1, n = 222)

Parameter

Average

Minimum

Maximum

Bicarbonate

187.4

77.5

237.8

Chloride

3.85

0.52

13.2

Sulfate

21.5

1.4

40.5

N03 (as N)

0.76

0.1

1.9

Calcium

44.9

16.5

57.1

Magnesium

12.9

1.9

18.5

Sodium

6.97

2.7

12.1

Potassium

1.78

0.7

2.6

TDS

282.7

113.4

357.8

Std. Deviation

26.5

1.6

6.2

0.4

5.8

2.0

1.2

0.3
35.9

Seasonal variations in water quality occur in the Clark Fork River and
nearby wells. The fluctuations water chemistry in these wells and the river
(Figure 6.4) are synchronous both temporally and in magnitude and, therefore,
provide confirmation that the aquifer is recharged by the river. At distances
over one half mile from the river, ground water quality remains fairly
constant with time at any one well (note wells MV16, MV27 and MV20 in Figure
6.4).

Metals are primarily found in the Clark Fork River and in wells near the
river. Copper is the exception and was detected randomly in about one third

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Figure 6.4 :

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« MV-18

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r«b-87

MV-20

Hydrographs showing seasonal variation in TDS for the Clark

Fork River and adjacent wells (top) and seasonal change In

TDS along a ground water flow path from CFNWB to MV20

(bottom).

of the wells throughout the valley. With one exception (MV38) , measured raetal
concentrations did not exceed drinking water standards. The Clark Fork River
had detectable levels of arsenic, zinc, and copper in November 1986, but at
other times all metals were at or below detection levels. Wells MV3, MV8 and
MVIO had elevated levels of zinc and copper. Well MV38 had a manganese
concentration of 0.17 mg/1 and was the only well sampled with manganese above
the detection level. These four wells are all within one quarter mile of the
Clark Fork River. Their elevated levels of metals may be related to
unidentified metal contaminated sediments in the Clark Fork River floodplain.
However, there are many other wells in close proximity to the river which do
not have detectable levels of metals. Concentrations of lead, mercury, and
cadmium are all at or below their detection levels.

Results of trace metal analyses should be viewed with caution and only as
trends because all sampled wells (except MV41 and MV42) had steel casings and
most samples were collected through the household plumbing. To reduce the
potential of raetal contamination, samples were collected only after the well
and piping system had been flushed with fresh ground water and the water
temperature and specific conductance had stabilized.

CHEMICAL TRENDS: RESULTS OF SPRING 1987 SAMPLING

Early in the project a review of aquifer stratigraphy and water quality
data indicated that the western portion of the aquifer may be susceptible to
contamination from individual sewage disposal systems. This area of the
aquifer was not well represented in the water quality sampling network and
therefore, an expanded quarterly sampling was carried out in May and June of
1987. Sixty-six additional domestic wells which were volunteered by their
owners for sampling were added to this round of sample collection (Figure
6.2). The resulting chemical data are extensive (Appendix 6B) and were used
to construct isoconcentrational maps of the aquifer and examine trends in
water quality.

Water quality steadily decreases in the down gradient direction from
Hellgate Canyon east and southeast toward the Bitterroot River. For example,
total dissolved solids increase from a low of about 240 mg/1 to a high of 358
mg/1 over distances of four to five miles. The distribution of total
dissolved solids in the Missoula Valley measured during spring 1987 (Figure
6.5) is very similar to that of the other four sampling periods. Comparison
of a potentioraetric map (Figure 5.2) and the isoconcentrational map (Figure
6.5) shows that the steady increase in total dissolved solids occurs in the
direction of ground water flow. The down gradient increase in total dissolved
solids is also apparent in Figure 6.4.

The observed down gradient increase in total dissolved solids is most
likely caused by natural dissolution in the aquifer of carbonate minerals,
such as calcite and dolomite. The increase in calcium, magnesium and
bicarbonate concentrations correlates with the increase in total dissolved
solids. Concentrations of other major ions, such as sulfate, sodium, and
chloride, do not correlate with total dissolved solids.

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Chloride concentrations in ground water are of interest because of the
potential impact on water quality from road salting and septic tank drain
fields. Average chloride concentration is 3.85 mg/1 (Table 6.2) and, as found
for total dissolved solids, the general trend is an increase in concentration
in the down gradient direction (Figure 6.6). Elevated concentrations occur Ln
several places. Two zones of higher chloride concentration appear north of
the Clark Fork River. One area is near the Russell Street Bridge and the
other is associated with well MWC30 (Figure 4.1). Both are probably
restricted to the north side of the river by the hydrologic divide created by
recharge from the river. The source of higher chlorides north of the river is
not known at this time. South of the river elevated concentrations of
chloride are found in near the Champion Mill and California Street. Septic
systems are the probable cause of part of the elevated chlorides found in this
area. A third area with elevated chlorides is located in the southern most
portion of the area between the Bitterroot River and well MWC14. In all
sampling periods well MWC14 had chloride concentrations of 10 to 12 mg/1,
levels which are about three times the average (Table 6.2). The higher
chloride could be originating form sewage disposal by household septic systems
or from natural recharge from the Tertiary sediments to the south. The final
area of elevated chloride Is in the western part of the valley, where some
concentrations about 25% above background were found. The sources of excess
chloride are most likely septic systems in areas which are not served by the
city sewer system (Ver Hey, 1987).

Nitrate concentrations exceeding 2.0 mg/1 were not detected in the wells
sampled as part of this study. Generally, nitrate concentrations increase in
the down gradient direction (Figure 6.7). Areas of apparent elevated nitrates
correspond with areas of elevated chloride. Based on the available data set,
the western portion of the aquifer has large areas with nitrate concentrations
between 0.8 and 2 mg/1. There are no known natural sources of nitrate in the
aquifer which can account for the levels found. It was originally thought
that storm water injection by over 2,000 storm drains in the urban area may be
loading nitrate into the system. However, Wogsland (1988) concluded that
storm water injection did not appear to be adding to nitrate to the system.
Ver Hey (1987) concluded, from a study of the affects of septic systems on the
ground water in the western portion of the aquifer, that ground water
contamination from both chloride and nitrate was occurring. Though this study
did not detect concentration over about 2.0 mg/1, other researchers have found
high concentrations of nitrate associated with some individual wells in the
western portion of the aquifer and in subdivisions in the southwestern portion
of the valley (Howard Newman, personal comm., 1988).

Sixty-six wells were sampled for dissolved trace metal concentrations.
All concentrations were below EPA drinking water standards (Appendix 5B).
Most concentrations were below instrument detection limits.

Water samples collected during the spring 1987 sampling period were
analyzed for coliform bacteria (Figure 6.8). Eighteen out of the 98 water
samples tested positive for bacteria. All but one of the positive samples
came from wells west of Reserve Street. The one positive sample east of
Reserve Street came from observation well MV34, which was never disinfected
after completion and has not been used for water supply.

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EPA priority pollutant scans were completed on samples from six wells
(Appendix 6C) . In the 112 analyses performed on each sample, all constituents
were below the instrument detection limit with the few exceptions listed in
Table 6.3. All detectable concentrations are lower than recommended drinking
water standards. The PVC casing in wells MV41 and MV42 probably caused the
contamination found in these wells. The origin of the methylene chloride (a
common laboratory solvent) in two of the samples is unknown.

TABLE 6.3
Results of EPA Priority Pollutant Scans

Well Compound above detection limit Concentration

(ug/1)

MV34 none

MV42 bis (2-ethylhexyl) phthalate 40

#41 bis (2-ethylhexyl) phthalate 80

//28 none

#33 methylene chloride 22

#60 methylene chloride 20

EXAMPLES OF GROUND WATER CONTAMINATION

This section illustrates the vulnerability of the Missoula Aquifer to
contamination by spills, leaks and area wide percolation of water from a
number of small sources. Brief summaries of research on a herbicide release,
the storm drainage system and septic tanks are presented.

A. Herbicide Release

In the fall of 1984, the Montana Department of Agriculture discovered
trace amounts of piclorara (0.052 ug/1) and 2,4-D (0.9 ug/1) in a well serving
the Missoula County Weed Control Facility (MCWCF) (Figure 6.9). They also
found picloram (2.4 to 4.5 ug/1) in two wells serving a commercial trailer
court and campground. The most likely source of these herbicides was the
MCWCF waste sump, which was found to be contaminated with picloram, bromacil,
2,4-D and 2,4,5-TP (Figure 6.10). Herbicides were also found in the soils
around and beneath the sump drain. The Montana Water Quality Bureau
determined that the herbicide levels found in area wells were orders of
magnitude lower than available drinking water standards but still recommended
additional analysis of the site.

Pottlnger (1988) assessed the ground water flow system and the transport
of herbicides from the MCWCF during 1985 and 1986. He found that picloram and
bromacil were the only herbicides found in measurable concentrations in an
area covering over one and one half square miles (Figure 6.11). The
distribution of the contaminants was controlled by water table variations of
over 20 ft and by a seasonal shift in the ground water flow direction from
south in the spring and early summer to southwest by west in midwinter. These
changes in the ground water system are caused by seasonal variations in

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General area in which measurable concentrations of picloram
and bromacil were detected in the ground water (Pottinger,
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discharge to the Missoula Aquifer from the Grant Creek Valley and from Grant
Creek as it flows into the valley over the Missoula Aquifer. Pottinger (1988)
successfully constructed a numerical ground water flow model accounting for
seasonal variation in recharge and developed a solute transport model to
predict the migration of the herbicides. Figures 6.12 and 6.13 show the
modeled position of the herbicide contaminated ground water in 1974 and 1989.

In summary, Pottinger (1988) demonstrated that contaminating the aquifer
at one small point (less that 100 ft^ in size) resulted in the spreading of
the contaminant over 1.5 rai^ . His work also showed that small portions of the
aquifer could be successfully modeled.

B. Storm Drain Recharge

Missoula has no valley wide storm sewer system. Instead, runoff from
streets percolates directly into the aquifer from over 2,000 street corner
drains. Wogsland (1988) measured the quality and quantity of urban runoff in
residential and commercial areas in the valley and then, using two sites
instrumented with soil water and ground water sampling devices, examined the
impact of storm water disposal on aquifer water quality (Figure 6.14).

Wogsland (1988) found that, with the exception of runoff from spring
snowmelt, street runoff entering the aquifer is low in total dissolved solids
and free of EPA priority pollutants. Commercial areas, however, do generate
runoff with water quality that is worse than that from residential areas. As
the water migrates through the vadose zone and towards the aquifer, it reacts
with earth materials and increases in mineral content (Figure 6.15). Runoff
from snowmelt runoff contains higher total dissolved solids and sodium
chloride than runoff from other sources. These higher concentrations appear
to diminish as the water passes downward through the vadose zone. Figure 6.16
shows the impact of the contaminated snowmelt runoff on ground water quality
directly beneath instrumented drains in a residential and a commercial area.
The chloride concentration at the commercial site exceeds background by over
four times during recharge by snowmelt. The data also show that the
concentration of iron in the ground water is usually below detection except
when chloride rich snowmelt waters reach the water table (Figure 6.17). This
implies chlorides may be facilitating the transport of metals in runoff and In
the vadose zone to the aquifer. Figure (6.18) shows the variation in nitrate
concentration in the ground water at the commercial site. The variations in
nitrate levels do not appear to be correlated with storm water injection but
appear to vary in response to changes in the regional flow system.

Wogsland (1988) also sampled existing wells In the commercial area
between South Avenue and US 93. Though her work is not conclusive, it appears
that total dissolved solids and metal concentrations are higher than
background levels in this part of the aquifer.

Wogsland's (1988) research documents the importance of snowmelt runoff
and dissolution of vadose zone materials in determining the water quality in
the aquifer. She concluded that the vadose zone supplies major ions to the
aquifer and attenuates metal migration except during spring snowmelt recharge.
Further installation of sump storm drains will contribute to minor aquifer

0.0021 mg/1

omuriKkmim

Figure 6.12: Modeled distribution of herbicides In 1974 (Pottlnger, 1988).

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Figure 6.13: Modeled distribution of herbicides in 1989 (Pottinger, 1988)

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time for a commercial site and residential site (Wogsland,
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contamination. But, more importantly, Missoula's storm drains provide an open
conduit to the aquifer for any contaminants or hazardous wastes spilled on
city streets.

C. Domestic Sewage Recharge i

Two household septic systems located in the western part of the valley
(Figure 6.19) were instrumented and studied in 1985 and 1986 by Ver Hey
(1987). An access port was installed in each septic tank and soil water
samplers were installed at two depths in the vadose zone below the drain -
field. Water meters were used to determine the rate at which waste entered
the septic system. Wells were used to sample the quality of the underlying
ground water (Figure 6.20). Both sites were located in coarse soils
classified as Grantsdale loam. All legal requirements for septic system
installation had been met at each site. The water table varied seasonally
between 8.5 and 14 ft below land surface.

The study showed that the septic drain fields are relatively ineffective
in treating household effluent due to the coarse grained soils in which the
systems were installed (Table 6.4). Approximately 200 gallons per day per
household enter each system, but the water retention time in the coarse soils
is so short that all effluent percolates out of the drain field within 15 ft
of the septic tank. Also no biological mat was found during excavation of the
drain fields. Water analyses show that no statistically significant reduction
in phosphorus or nitrogen concentrations occurs as the effluent percolates
from the drain field to the water table. A plume of nitrate and chloride
could be traced down gradient from the drain field at both sites (Figure 6.21
and 6.22). Fecal coliform numbers were above the maximum detectable limit in
ground water samples taken directly below the drain field and were still high
(eight bacteria per 100 ml) 50 ft down gradient (Figure 6,23). The presence

TABLE 6.4

Septic System Treatment as Mean Per Cent Removal of Effluent

Constituents, Year Two (Ver Hey, 1987)

Specific
Conductance

Total Ortho-
Nitrogen Phosphate

Sodium Chloride

Site H

Septic System

nt*

Vadose Zone

-21

Aquifer

Site T

. ,

Septic System nt
Vadose Zone nt
Aquifer nt

35
nt
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nt
nt
46

nt
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nt

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nt
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of a 0.2 inch amphipod in a ground water sample suggests that void spaces are
large in the underlying aquifer. Samples of ground water taken up gradient of
the study site have elevated concentrations of chemical constituents and
possibly indicate wide spread degradation of ground water quality by
individual septic systems in the area.

Ver Hey (1987) concluded that traditional septic system design is
ineffective in treating domestic waste in the coarse alluvial soils of the
Missoula Valley. She cautioned that the continued use of existing systems and
the addition of new systems would result in degradation of existing ground
water supplies.

103

CHAPTER 7
GROUHD WATER FLOH MODELING

INTRODUCTION

Construction and calibration of a transient model of ground water flow
through the Missoula Aquifer proved to be a difficult task. Although
significant progress was made in defining the distribution of aquifer
properties and boundaries, and in understanding the interaction between the
Clark Fork River and the ground water system, we were unsuccessful in
developing a complete numerical model which could be used for predictive
purposes. Future modeling efforts based on this work and coupled with
collection of additional data will be able to describe the responses of the
aquifer to variations in Clark Fork River recharge and to pumping by Mountain
Water Company. The following discussion of ground water modeling is taken
from unpublished work by Brick (1987).

MODEL DESIGN

A two-dimensional model was developed for ground water flow in the
aquifer for one year starting in October 1985 and ending in September 1986.
The area covered is shown in Figure 7.1. The finite difference computer code
named PLASM developed by Prickett and Lonnquist (1971) and subsequently
adapted by Prickett for the IBM PC series computer was used. Several
modifications were made to this program to tailor it to this study. Data for
the model were taken from past and ongoing work, especially that of Clark
(1986) and Pottinger (1988).

The Missoula Valley and the grid for this model are bounded by Mt.
Sentinel to the east. Waterworks Hill to the north, the South Hills and
Bitterroot River to the south and thick Lake Missoula sediments of low
permeability to the west. The grid consists of 38 columns and 31 rows with
variable node spacing. The density of nodes is greater over the population
center of Missoula and the locations of municipal wells (Figures 7.1 and 7.2).
The boundary conditions for the grid are illustrated in Figure 7.3. The east
and west boundaries are no-flow boundaries. A constant head is assigned to a
portion of the north boundary because flow maps and previous work by Geldon
(1979) indicate a small source of water from the north, possibly from the
Tertiary sediments. Pottinger (1988) also used a constant head boundary in
this area. The model was modified to allow the constant head at the north
boundary to change seasonally because a seasonal change in head is indicated
by potentiometric maps of the aquifer (Figures 5.1 to 5.3). The constant head
at this boundary was set to vary between 3,142 ft and 3,155 ft. The south
boundary is defined by the Bitterroot River and is assigned a constant head
determined by the river's stage and gradient. This boundary is discussed
further below.

Apart from the model's boundaries, initial input parameters include the
storage coefficient, hydraulic conductivity and the thickness of the aquifer.
In each of the two modeled dimensions, the aquifer is divided into blocks

104

Figure 7.1: Grid for finite difference model (after Brick, 1987)

105

Figure 7.2: Model grid and location of Mountain Water Company wells
(after Brick, 1987).

106

Grant Creek

North boundanrconstant head

Rattlesnake Creek

constant head

Figure 7.3: Model boundary conditions and locations of wells used to
evaluate calibration (after Brick, 1987).

107

which are bounded by two adjacent nodes and which have constant hydraulic
properties - Heterogeneity can be modeled by assigning different hydraulic
conductivity and storage values to each of these blocks. Hydraulic
conductivity and storage coefficient cannot vary with depth because the model
Is two-dimensional. The storage coefficient in this model is 0.10, which is
close to the 0.12 value measured in permeameter tests and to storage
coefficient values (0.03 to 0.10) determined from several aquifer tests
(Clark, 1986),

The hydraulic conductivity distribution used in the model was based on
previous research and Figure 3.8. The model itself was also used to evaluate
hydraulic conductivity values. The final distribution of values is
illustrated in Figure 7.4. It should be emphasized that this distribution of
hydraulic conductivity values was chosen because the results of the model fit
known steady state water level data and not because it necessarily represents
reality.

The bottom elevation of the modeled aquifer is represented by the contour
lines in Figure 7.5. This configuration is based on the work of Morgan (1986)
and Figure 3.2.

Additional input parameters include the location and temporal variations
of major water sources and sinks. These include the withdrawal from municipal
wells and infiltration from streams (Clark Fork River, Rattlesnake Creek and
Grant Creek) . The selection of Clark Fork River recharge rates is discussed
below. Rattlesnake Creek and Grant Creek are treated as Injection wells where
they enter the valley. The amount of water injected was determined from the
work of Sendler (1986) and du Breuil (1983). Rattlesnake Creek is represented
by one injection well while Grant Creek is represented by eight wells which
extend into the valley. The monthly injection schedules for these wells are
in Table 7.1. These sources provide a minimal amount of water except during
runoff in May and June.

TABLE 7 . 1

Schedule of Injection Wells Used to Simulate

Rattlesnake Creek and Grant Creek

(gpd per node)

Month Rattlesnake Creek Grant Creek

Oct 10,000 50,000

Nov 10,000 50,000

: . Dec 10,000 50,000

Jan 10,000 50,000

Feb 50,000 100,000

Mar 100,000 400,000

Apr 200,000 600,000

May 300,000 900,000

Jun 400,000 900,000

Jul 100,000 200,000

Aug 10,000 100,000

Sep 10,000 200,000

108

Figure 7.4: Distribution of hydraulic conductivity used in the model
(after Brick, 1987).

109

o
o

-• — •— • — »-

« m

cs
o

■ »■■ ■ ■•■■-■< — » — • — •-

« « • «

X5
lor " •

'(Or

1 mi

CONTOURS IN FT

• ■ • < ■»'

Figure 7.5: Elevations of the base of the aquifer used in the model
(after Brick, 1987).

110

Pumping schedules for municipal wells were obtained from Mountain Water
Company. The monthly production rates were converted to a daily rate and then
divided in half to account for the assumed 50 percent loss by leakage from
distribution lines (Table 7.2). This leakage recharges the aquifer, and, for
all practical purposes, reduces pumpage by half. MWC well locations are shown
on the modeling grid in Figure 7.2.

TRANSIENT FLOW SIMULATION

Downward leakage of Clark Fork River water is the main source of recharge
to the Missoula Aquifer and therefore the primary driving force of the ground
water flow system. The recharge rate is dependent on the stage of the river
and the leakage rate through the river bed. The flow system responds less
strongly to the stage of the Bitterroot River, discharge from Grant,
Rattlesnake Creeks and pumping of large yield wells and variation in lateral
flow from Tertiary and Frecambrian bedrock which bounds the aquifer.

In order to model the aquifer over a one year period, it was necessary to
periodically change the stage of the Clark Fork and Bitterroot Rivers and to
change the leakage rate of the Clark Fork River. To accomplish these
conditions, the PLASM code was modified to read new values of head (stage) and
leakance each month. Average monthly discharges of the Clark Fork River were
calculated from the USGS gaging data recorded at the station above Missoula
(Figure 2.3). Monthly discharges of the Bitterroot River were estimated by
subtracting Clark Fork River discharge measurements at the gaging stations
above and below Missoula. These values compared favorably to several
discharge measurements made on the Bitterroot River by Clark (1986).
Discharges were converted to river depth or stage (Figure 7.6) using stage-
discharge relationships developed for each river by Clark (1986). River stage
elevations were then correlated with river profiles determined from USGS
topographic maps.

The leakage of the Clark Fork River bed is known to vary, possibly in
response to seasonal changes in the bed's permeability (Clark, 1986). Leakage
was calculated from Clark's measurements of discharge lost from the river.
Though estimated in Chapter 5, the average rate of leakage is not known for
each month. Based on available data, leakage rates from 0 to 5 gpd/ft-^ at
each river node of the model were selected to initiate modeling. Since the
value of leakage was not well determined, a number of different values were
tested in the model. The final values used are in Table 7.3.

The transient model simulation begins in October 1985 after the model has
reached steady state conditions. The steady state heads are used as initial
heads for October. The model then reads new head and leakance values for the
Clark Fork River, new head values for the Bitterroot River, new injection
rates for wells simulating Grant Creek and Rattlesnake Creek, new discharge
rates for the pumping wells, and adjustments for recharge from the northern
model boundary. A new set of simulated heads is then calculated for the end
of the month. This process continues for 11 months until completion of the
simulation at the end of September 1986.

CLARK FORK RIVER AVERAGE MONTHLY STAGE

AT WALKINO BR1D0E, CFNm

S.I 21

S.114

118ft- 1181

BITTERROOT RIVER AVERAGE MONTHLY STAGE

AT BVCXHOUSE BRIDGE. BBBR

B.178
8.1778 -
8.1778 -
8.1774 -
8.1772 -

8.177 -
8.1788 -

t'J 3.1788 -
g I 8.1784-
5 I 3.1782 -
£ 8.178 -
8.1788 -
8.1788
8.1784 H
8.1782

8.178 -
8.1748 -
8.1748

Figure 7.6

Hydrographs showing average monthly stage used in the model
for the Clark Fork River (top) and the Bitterroot River
(bottom) (Brick, 1987).

112

TABLE 7.2

1985-1986 Pumping Schedules for

Mountain Water Company Municipal Wells

(gpd)

MWCl

MWC2 MWC3,37

MWC7

MWC8

MWC9

MWCIO

Oct

56307

398742

2278655

62339

Nov

34049

604226

636263

59597

Dec

348403

581033

162

Jan

412968

641031

129

182629

Feb

275565

609965

Mar

293903

677771

Apr

746774

655636

162

44291

May

618194

50918

1262532

659812

25097

3113

653807

Jun

653791

200184

279307

671618

88194

13355

639984

Jul

503758

161202

1192516

677912

92548

2500

634226

Aug

530081

198329

1115790

621865

103887

13613

647484

Sep

187145

3608

655852

630710

MWCll

MWCl 4

MWCl 6

MWC18

MWCl 9

MWC20

MWC21

Oct

607403

307

145

113

Nov

25920

574452

129

145

Dec

19049

129

145

613

226

Jan

680420

586855

145

129

62371

35694

Feb

903210

530984

56533

35694

Mar

1004435

557129

162

194

37065

145

Apr

968452

567710

371

160087

May

994742

556726

42178

40129

364645

304258

Jun

684984

575242

142952

5678

84774

433516

325645

Jul

323887

589178

104307

3016

99420

426178

455339

Aug

986016

594694

182984

9258

190920

516710

904597

Sep

346307

511484

61403

55403

MWC22 MWC23,24

MWC25

MWC26

MWC27

MWC29

MWC30

Oct

265008

10420

98839

1791

Nov

251123

14742

90920

113

Dec

266299

13710

162

95791

1016

Jan

231207

210

110662

2304920

Feb

1065

219426

100420

2062322

Mar

6033

250621

920

119887

2307322

Apr

11710

567710

8420'

134242

2200161

May

16516

326033

82565

523581

11484

192081

2275193

Jun

62194

347307

247065

478629

50291

273145

1566242

Jul

49484

343581

239033

370371

20452

279339

728452

Aug

60533

369820

322420

731984

43920

335355

996274

Sep

162

282431

13500

43629

117662

59807

113

MWC31

TABLE 7.2, continued
MWC32 MWC33 MWC3A

MWC35

MWC36

Oct

2404516

1628129

226

3530806

194

3113

Nov

2323500

1155371

274

3525967

178

Dec

2406435

1070435

323

3826613

274

Jan

14871

1555742

237758

3031935

161145

355

Feb

4242

1478952

4242

3061774

113

468

Mar

10645

1692774

291

3550806

113

952

Apr

564678

1265435

145210

3023226

6371

12936

May

586420

1783887

752855

1165000

161420

77645

Jun

1765032

2039742

1074580

3475645

234936

Jul

2413258

2031677

1050338

4040484

178

303750

Aug

2387984

2078258

1231355

3735161

162

644581

Sep

2356532

1794613

68936

3464193

162

18081

TABLE 7.3

Leakage Values Used in the Model

(gpd/ft3)

Oct

Apr

2.0

Nov

May

5.0

Dec

Jun

5.0

Jan

Jul

1.0

Feb

Aug

Mar

1.0

Sep

MODEL RESULTS

Despite numerous attempts and manipulations to the model, the simulation
is still not calibrated to the actual head data. It provides a good, general
overview of ground water flow in the valley but is not sufficient for
predictive modeling.

Examples of the distribution of heads generated by the model are
illustrated in Figures 7.7 to 7.9. Figures 7.10 to 7.11 are hydrographs which
illustrate the modeled change in head over time compared to actual measured
values at selected locations on the grid.

The transient simulation is not a
water table. The model more accurately
of the aquifer than in others. For ins
7.11) shows the correct fluctuation pat
simulated heads is one to two feet high
other problem areas are shown in Figure
show simulated heads fluctuating either
to measured heads. Another problem in

perfect reproduction of the actual

simulates actual heads in some regions
tance, the hydrograph for MV22 (Figure
tern but the overall elevation of
er than actual heads. Examples of
s 7.10 and 7.11. These hydrographs

too much or not enough in comparison
the model is the head distribution near

114

OCTOBER

WATER TABLE CONTOURS, FTf 3000
MODELED
MEASURED

Figure 7.7: Modeled and measured water table elevations, October 1985
(after Brick, 1987) .

115

MARCH

WATER TABLE CONTOURS, FTf 3000
MODELED
MEASURED

Figure 7.8: Modeled and measured water table elevations, March 1986
(after Brick, 1987).

116

JUNE

WATER TABLE CONTOURS, FTf 3000
MODELED
MEASURED

Figure 7.9: Modeled and measured water table eleva
Brick, 1987).

tions, June 1986 (after

117

WELL MV-3

1S5

^•%

o
o
o
n

z
o

modalad raaults

-4- fl«ld m«amir«m«nfa

WELL MV-31

140

139 -

13B

^ 137 -

o
o
o
n

4, 136 -

135

134

133

132 -1

mod«l«d rcaulta

flald mMiMiranMnti

Figure 7.10: Hydrographs showing modeled and measured heads at wells MV3
(top) and MV31 (bottom) (Brick, 1987).

118

WELL MV-22

o
o
n

«^

z
o

116

115

110 -f

D medalad raiuKa

•*■ fl«ld m«aturwn«nla

Figure 7.11: Hydrographs showing modeled and measured heads at well MV22
(Brick, 1987).

119

Grant Creek. Although the model injects more water than is really warranted
by the data, the heads in this area are consistently too low.

The spacial distribution in modeled heads matches measured data best in
March (Figure 7.8). Modeled water tables for October and June are poor
representations of the actual head distribution (Figures 7.7 and 7.9) One of
the problems encountered was that the model fails to simulate the wide
seasonal and spacial changes in the water table illustrated in Figure 5.5.
This figure shows the migration of the 3,135 ft water table contour from June
to March, a trend not reproduced by the model.

The model is imperfectly calibrated largely because the necessary input
parameters are imperfectly known on a valley-wide scale. The Missoula Aquifer
is a large heterogeneous aquifer with many sources and sinks of water.
Although many parameters have been quantified for the Missoula Aquifer, more
continuous data are needed to effectively use the model. To date, missing
data have been filled by approximations. In particular, a better picture of
the distribution of hydraulic conductivity is needed. Figure 3.8 shows a wide
range of conductivity in the aquifer, however, the actual distribution of the
various fluvial sediments comprising the aquifer is poorly known. Morgan's
(1986) effort to delineate the distribution of hydraulic conductivity was the
most detailed but the problem needs to be considered in more detail.
Evaluation of the aquifer using a geologic facies model is needed. However,
extensive drilling and sampling of the aquifer would be required to build a
detailed model.

Until more data are obtained from the aquifer, the best approach toward
modeling may be to work at a different scale. Smaller segments of the aquifer
have been modeled successfully (Pottinger, 1988; Peery, 1989) as the problems
of large scale heterogeneity are less acute. Smaller models may ultimately be
more useful for analyzing specific problems and making specific conclusions.
As more information becomes available it should be possible to eventually
model the entire valley.

120

CHAPTER 8
CONCLUSIONS AND RECOMMENDATIONS

The results of this two year study lead to the following conclusions:

1. The Missoula aquifer is stratigraphically complex. It is composed
of three units, the first or second of which is not always present. The
upper most unit, Unit One, is 10 to 40 ft thick and is composed of
interbedded boulders, cobbles and gravel. The middle zone. Unit Two, is
composed of up to 40 ft of tan to yellow silt with sand and gravel.
Unit Three, the basal unit, is composed of 50 to 100 ft of interbedded
gravel, sand and silt.

2. The hydrologic properties of the aquifer reflect site specific
stratigraphy and depositional environments. Values of hydraulic
conductivity and transmissivity appear to decrease south and southwest
of the Hellgate Canyon and Grant Creek area. The hydraulic conductivity
of Unit One and Unit Three ranges from 10,300 gpd/ft^ to 25,000 gpd/ft^.
Values for Unit Two average 200 gpd/f t^ . Transmissivity values for Unit
One and Unit Three range from 103,000 gpd/ft to 1,710,000 gpd/ft.
Values for Unit Two average 8,000 gpd/ft. Approximate hydraulic
properties of the entire aquifer, assuming the aquifer is acting as one
homogeneous unit, are a porosity of 0.20, specific yield of 0.12,
hydraulic conductivity of 18,200 gpd/ft^ and a transmissivity of
1,152,000 gpd/ft. -:

3. Changes in aquifer storage are indicated by water table variations.
These fluctuations reflect seasonal changes in the quantity of recharge
reaching the aquifer and in the rates of ground water withdrawal by
pumping and by natural discharge. During 1985-1986 and 1986-1987 wells
closest to the influent portions of the Clark Fork River, Rattlesnake
Creek and Grant Creek showed water table fluctuations of three to 17
feet. Wells located further from these points of recharge fluctuated at
a lower amplitude. During 1985-1986, peak water table elevations
throughout the valley were higher than peak elevations in 1986-1987.
The water table low recorded in late winter was lower in 1987 than the
previous year. These valley wide trends appear to have begun in 1983,
when climatic conditions apparently changed, resulting in less than
normal Clark Fork River spring discharge and aquifer recharge. Also
since 1983, the main water producer in the valley. Mountain Water
Company, more than doubled its pumping rate in order to increase its
production of ground water to supply 100 percent of its needs. A step
function decline in water level occurred at those Mountain Water Company
wells which experienced the increased pumping.

4. The Clark Fork River is a losing stream and recharges the aquifer
from four to six miles of its channel during much of the year. Mass
balance calculations show that the river accounts for 90 percent of
aquifer recharge. Total aquifer recharge is 15 times greater than

121

withdrawal from Mountain Water Company wells, Clark Fork Water Company
wells and approximately 4,700 individual wells. Based on these data,
the apparent valley wide decline in the water table since 1983 is a
result of a reduction in recharge caused principally by lower than
normal flow in the Clark Fork River.

6. The water quality of the aquifer is good and does not require
treatment prior to use in most areas. The water is dominated by calcium
and bicarbonate ions, is low in total dissolved solids (less than 500
mg/1) and is similar in chemistry to the Clark Fork River. Wells
located adjacent to influent streams reflect the chemistry of these
sources of recharge. As the ground water flows away from these recharge
areas, total dissolved solids increase through pollution and aquifer
dissolution. The presence of chloride and nitrate concentrations which
are elevated above natural background levels suggests degradation from
anthropogenic sources is occurring. Injection of storm water was found
to increase chloride concentrations at the water table beneath storm
drains. The disposal of sewage by seepage rings or drain fields appears
to be degrading water quality in portions of the valley as both nitrate
and chloride concentrations are elevated in areas served by septic
systems. Fortunately, concentrations do not exceed drinking water
standards. However, coliform bacteria and fecal coliform bacteria have
been found in individual water supplies at levels which exceed drinking
water criteria. The aquifer is extremely vulnerable to contamination
because it is unconfined and consists of generally coarse material.

7. An attempt to numerically simulate the aquifer's complex
stratigraphy and recharge-discharge relationships was unsuccessful. The
computer model could not be calibrated using field data and then
independently verified with a second set of field data. Problems were
viewed as being partially attributable to inadequate definition of
recharge rates from the Clark Fork River and of the distribution of
aquifer properties.

The conclusions listed above imply that the Missoula Aquifer is an
aerially extensive, thin, unconfined system which is geologically complex and
vulnerable to contamination. It is Missoula's sole source of water and an
irreplaceable resource. Protection of the Missoula Aquifer requires
continuing long term observation and management. The following
recommendations are based on the conclusions outlined above:

122

1. Maintain a long term water level observation network. Analyses of
historical water table fluctuation data were critical in deciphering the
relationships between apparent water level declines and causes for such
trends. However, previous data collection efforts failed to measure
true annual minima and maxima of the water table so trends were
difficult to define. Secondly, years of daily water level records
extending over a number of years with differing climatic conditions are
needed to calibrate and verify a numerical model of the system. The
establishment of a water level monitoring network has been initiated as
part of this project. Nine wells drilled specifically for water level
trend observation were installed. Seven of these wells and well MV31
(MWC7) have been equipped with electrical transducers and electronic
data storage systems. The existing system should be expanded by
installing similar water level recording devices at the remaining two
wells, MV41 and MV42. In addition, water level records for the region
north of the Clark Fork River should be reviewed, and one to two wells
in this area should be instrumented with similar water level recording
systems. MWC wells would be suitable if permission for installation can
be arranged. Funding is required to maintain the equipment, collect
monthly manual water level readings and to reduce and report the data.

2. Along with a water level monitoring network, a water quality
monitoring network should be established. Approximately 30 wells
located throughout the aquifer and finished in various aquifer units
should be sampled four times a year. Water quality analyses should
include gross ions, metals and metaloids, and coliform bacteria.
Yearly, selected wells should be sampled for organic analyses, including
volatiles, pesticides and solvents. The results of this sampling would
be used to identify trends in water quality and provide a baseline from
which the effects of point and non-point contaminant sources can be
evaluated. Funding is required to pay for water quality analyses,
sample collection and equipment, and data reduction and reporting,

3. The development of a numerical model of the aquifer should be
continued. Long term management of the aquifer as the sole source of
water supply for Missoula and surrounding areas requires the ability to
test the effects of increased pumping and changes in recharge rates
prior to their occurrence. A calibrated and verified model will provide
aquifer managers with such a capability. In addition to water supply
management, a model could be used to predict pathways and directions of
contaminant migration and to identify sensitive portions of the aquifer
which require special management consideration. Developing a
computerized model will require additional data for calibration
including water level records collected over the last year and better
estimates of the distribution of hydraulic conductivity and Clark Fork
River leakage rates. Aquifer testing of 60 to 100 wells should be
conducted to provide an independent check on the thousands of pumping
and drawdown data recorded by drillers. Additional mass balance work to
estimate Clark Fork River recharge should be attempted. Funding will be
required to refine and improve modeling efforts, to support additional
data collection and to maintain and update the model once it is
operating.

123

It is the goal of these recommendation to provide the citizens of Missoula
with the facts needed to make educated management decisions regarding the
future of their source of potable water. Halting data collection and further
work on model development would be short sighted and could lead to poor
planning and crisis situations in managing this critical resource.

124

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125

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Thesis, Dept. of Geology, unpublished, 133 pp.

127

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Full text of "Missoula Valley aquifer study : hydrogeology of the eastern portion of the Missoula aquifer, Missoula County, Montana"

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